Comprehensive Report on the Investigational Compound DB01721: An Indinavir Analogue Targeting HIV-1 Protease

I. Introduction and Compound Identification

A. Overview of DB01721 as an Experimental Small Molecule Drug Candidate

The compound identified by DrugBank accession number DB01721 is an experimental small molecule that has garnered attention primarily for its structural relationship to Indinavir, an established Human Immunodeficiency Virus (HIV) protease inhibitor.[1] As an analogue of Indinavir, DB01721 has been a subject of research aimed at understanding and potentially overcoming the pervasive challenge of drug resistance in HIV-1 therapy. Its investigation is highlighted by detailed structural and biochemical analyses, particularly its interaction with multi-drug resistant (MDR) variants of the HIV-1 protease enzyme.[2]

The study of DB01721 is emblematic of a critical strategy in antiviral drug development: the modification of existing, clinically successful drugs to address limitations such as emergent viral resistance or suboptimal pharmacological profiles. Indinavir, while a potent therapeutic, has seen its utility diminished by the selection of resistant HIV-1 strains and a demanding clinical regimen.[5] The development of analogues like DB01721, which incorporates specific dual modifications to the Indinavir scaffold, represents a rational medicinal chemistry approach. The evaluation of such analogues against MDR HIV-1 protease aims not only to identify new therapeutic candidates but also to deepen the understanding of structure-activity relationships (SAR) and the molecular mechanisms by which resistance can be circumvented. The detailed characterization of DB01721, particularly the finding that its combined modifications did not lead to improved potency against a resistant protease [3], provides valuable insights into the complexities of inhibitor design and the adaptive capabilities of viral targets. This iterative process of design, synthesis, and testing, even when not yielding a superior drug candidate, contributes significantly to the knowledge base required for developing next-generation antiviral agents.

B. Primary Chemical Name and Identifiers

The primary systematic chemical name for DB01721, consistent with its structural features as an Indinavir analogue with specific modifications, is N-[2-hydroxy-1-indanyl]-5-[(2-tertiarybutylaminocarbonyl)-4(benzodioxol-5-ylmethyl)-piperazino]-4-hydroxy-2-(1-phenylethyl)-pentanamide.[4]

Key database identifiers include:

• DrugBank ID: DB01721 [1]
• PubChem Compound ID (CID): 5496642 [7]

The consistency between the systematic chemical name and the structural details of the research compound, particularly known as XN2 or XN1336-52 in pivotal studies [4], is crucial for accurate data aggregation. The "benzodioxol-5-ylmethyl" moiety within the systematic name corresponds to the methylenedioxyphenyl group at the P2' position of the inhibitor. Similarly, the "2-(1-phenylethyl)-pentanamide" fragment describes the P1 side chain containing an s-methyl group addition to the benzylic carbon, distinguishing it from the simple phenylmethyl (benzyl) group in Indinavir. This structural correlation across various nomenclature systems and database entries (DrugBank DB01721, PubChem CID 5496642, and the PDB ligand XN2) ensures that this report accurately synthesizes information pertinent to this specific chemical entity.

C. Key Synonyms and Research Codes

DB01721 is referenced by a variety of synonyms and research codes across different scientific databases and publications, reflecting its investigational nature. A consolidated list is essential for comprehensive information retrieval:

• Analogue of indinavir drug: A descriptive term used in DrugBank.[1]
• XN2: The three-letter ligand identifier in the Protein Data Bank (PDB) for entry 1k6v, which details the crystal structure of this compound bound to HIV-1 protease.[2]
• PD060269: An identifier used in PubChem and other chemical databases.[7]
• 1k6v: Refers to the PDB entry (deposition code) containing the XN2 ligand.[2]
• XN1336-52: The internal research code used in the primary publication by Schiffer C.A. et al. (2002), Protein Science, which describes the structural and biochemical analysis associated with PDB entry 1k6v.[4]
• NS00071696: An additional identifier found in PubChem and the COCONUT natural products database.[7]
• Q27467591: The Wikidata item identifier.[7]
• CHEMBL1257519: A ChEMBL identifier. While the provided research material did not contain direct bioactivity data linked to this specific ChEMBL ID for DB01721 [12], ChEMBL is a critical resource for such information.

Table 1: Compound Identifiers for DB01721

Identifier Type	Value	Source/Context
Systematic Name	N-[2-hydroxy-1-indanyl]-5-[(2-tertiarybutylaminocarbonyl)-4(benzodioxol-5-ylmethyl)-piperazino]-4-hydroxy-2-(1-phenylethyl)-pentanamide	User Query; Consistent with Schiffer et al. (2002) structure for XN1336-52
DrugBank ID	DB01721	User Query, DrugBank 1
PubChem CID	5496642	PubChem 7
PDB Ligand ID	XN2	Protein Data Bank (for PDB entry 1k6v) 2
PDB Entry (ligand present)	1k6v	Protein Data Bank 2
Research Code (Schiffer et al.)	XN1336-52	Schiffer et al., Protein Science (2002) 4
Other Database ID (e.g., PubChem)	PD060269	PubChem, Probes-Drugs 7
Other Database ID (e.g., PubChem)	NS00071696	PubChem, COCONUT 7
Wikidata ID	Q27467591	Wikidata 7
ChEMBL ID	CHEMBL1257519	ChEMBL (cross-reference)
Descriptive Synonym	Analogue of indinavir drug	DrugBank 1

This table serves as an essential cross-referencing tool, enabling researchers to accurately identify and consolidate information about DB01721 from diverse scientific literature and databases, thereby preventing confusion that can arise from the use of multiple identifiers for the same experimental compound.

II. Chemical Characterization

A. Molecular Formula and Molecular Weight

The elemental composition of DB01721 is defined by the molecular formula C39​H50​N4​O6​. This formula is consistently reported across major chemical databases, including PubChem [7] and the COCONUT database.[8]

Based on this formula, the computed molecular weight is approximately 670.8 g/mol, as provided by PubChem.[7] More precise calculations yield a monoisotopic mass of 670.37303533 Da (PubChem [7]) and a similar value of 670.37304 Da from the COCONUT database.[8] The Probes-Drugs database also lists a molecular weight of 670.37 for the synonym PD060269.[11] The uniformity of these values across different sources provides high confidence in the basic chemical identity of the compound.

B. Detailed Chemical Structure

The precise atomic arrangement and bonding of DB01721 are captured by various chemical notation systems:

• IUPAC Name (Computed by PubChem/Lexichem TK 2.7.0): (2S)-4-(1,3-benzodioxol-5-ylmethyl)-N-tert-butyl-1-carbamoyl]-5-phenylhexyl]piperazine-2-carboxamide.[7] The "5-phenylhexyl" portion of this name refers to the P1 side chain, which incorporates the (1-phenylethyl) moiety specified in the query name and corresponds to the s-methyl group addition at the benzylic position discussed in the primary literature.[4]
• SMILES (Canonical, PubChem Computed): C[C@H](C1=CC=CC=C1)[C@H](C[C@@H](CN2CCN(C[C@H]2C(=O)NC(C)(C)C)CC3=CC4=C(C=C3)OCO4)O)C(=O)N[C@@H]5[C@@H](CC6=CC=CC=C56)O.[7] This Simplified Molecular Input Line Entry System string unambiguously defines the molecule's connectivity and stereochemistry, clearly representing the P1 s-methylbenzyl group and the P2' methylenedioxyphenyl (benzodioxol-5-ylmethyl) group.
• InChI (PubChem Computed): InChI=1S/C39H50N4O6/c1−25(27−10−6−5−7−11−27)31(37(46)40−36−30−13−9−8−12−28(30)19−33(36)45)20−29(44)22−43−17−16−42(23−32(43)38(47)41−39(2,3)4)21−26−14−15−34−35(18−26)49−24−48−34/h5−15,18,25,29,31−33,36,44−45H,16−17,19−24H2,1−4H3,(H,40,46)(H,41,47)/t25−,29+,31+,32+,33−,36+/m1/s1.[7] The International Chemical Identifier provides a standardized, unique textual representation of the molecule, including its stereochemical details.
• InChIKey (PubChem Computed): MJIRDPUZGGHJMX-OIVSQUILSA-N.[7] This is a hashed, fixed-length version of the InChI, useful for database lookups and verification of chemical identity.
• 2D and 3D Structures: Two-dimensional representations and computed three-dimensional conformers are available on PubChem.[7] Crucially, the experimentally determined three-dimensional structure of DB01721 (as ligand XN2) in complex with its biological target, HIV-1 protease, is archived in the Protein Data Bank under the entry code 1k6v.[2]

C. Stereochemistry and Isomeric Information

DB01721 is a chiral molecule possessing multiple stereogenic centers. The precise three-dimensional arrangement of atoms is critical for its biological activity, as interactions with enzyme active sites are highly stereospecific. The InChI string, with its stereochemical layer /t25-,29+,31+,32+,33-,36+/m1/s1, explicitly defines the absolute configuration at each of these chiral centers.[7] Similarly, the full IUPAC name incorporates stereochemical descriptors such as (2S), (1S,2R), etc., to denote these configurations.[7] The canonical SMILES string also encodes this chirality using @ (counter-clockwise) and @@ (clockwise) notations for tetrahedral stereocenters.

The defined stereochemistry of DB01721 is not an arbitrary feature but a fundamental determinant of its interaction with the HIV-1 protease active site. HIV-1 protease, being an enzyme, possesses a chiral active site exquisitely shaped to bind its natural peptide substrates, which also have specific stereochemistry.[5] As a peptidomimetic inhibitor, DB01721 must conform to these stereochemical requirements to achieve effective binding and inhibition. Any deviation in the configuration at one or more of its chiral centers would likely lead to a significant loss of binding affinity and, consequently, biological activity. This underscores the importance of stereocontrolled synthesis and chiral purity in the development and study of such enzyme inhibitors. The specific stereoisomer detailed in the databases and co-crystallized in PDB entry 1k6v is understood to be the pharmacologically relevant form.

D. Physicochemical Properties (Computed and Predicted)

A comprehensive set of physicochemical properties for DB01721 has been computed or predicted by various chemoinformatic tools, primarily available through PubChem [7], DrugBank [1], and the COCONUT database.[8] These properties offer insights into its potential behavior as a drug molecule:

• Lipophilicity: The partition coefficient (LogP) is a key measure. XLogP3 is reported as 4.[7] DrugBank provides predicted LogP values of 4.37 and 3.94.[1] The COCONUT database lists an AlogP of 3.76.[8] These values consistently indicate that DB01721 is a moderately to highly lipophilic compound.
• Hydrogen Bonding Capacity: The molecule has a predicted Hydrogen Bond Donor Count of 4 and a Hydrogen Bond Acceptor Count of 8.[1] These values are within ranges typical for many orally administered drugs.
• Molecular Flexibility: The Rotatable Bond Count is predicted to be 11 or 12 [1], suggesting considerable conformational flexibility, which can be important for adapting to the binding site of a target protein.
• Polarity: The Polar Surface Area (PSA) or Topological Polar Surface Area (TopoPSA) is calculated to be 123.6 Ų.[1] This value is often correlated with membrane permeability and oral bioavailability.
• Molecular Size and Shape: DB01721 contains 6 rings in total, of which 3 are aromatic.[1]
• Aqueous Solubility: The predicted water solubility is low, at 0.0233 mg/mL.[1] DrugBank also reports a predicted LogS (logarithm of molar solubility) of -4.5 [1], consistent with poor aqueous solubility.
• Ionization Properties: The predicted pKa for the strongest acidic group is 13.03, while for the strongest basic group it is 6.9. The predicted physiological charge at pH 7.4 is +1.[1]
• Drug-Likeness Assessment:
• Lipinski's Rule of Five: DB01721 is predicted to violate this rule.[1] The primary reasons are its molecular weight (approximately 670.8 g/mol, exceeding the <500 Da guideline) and its LogP value, which is close to or exceeds the <5 guideline depending on the calculation method.
• Other Filters: It is predicted not to pass the Ghose Filter or Veber's Rule, but to pass the MDDR-like Rule.[1]
• Natural Product (NP)-likeness Score: A score of -0.29 [8] indicates that the compound is less like a natural product, which is expected for a synthetic drug analogue derived from Indinavir.

The physicochemical profile of DB01721, particularly its high molecular weight and significant lipophilicity leading to violations of common drug-likeness rules like Lipinski's Rule of Five, is a characteristic shared by many HIV protease inhibitors.[5] These properties often arise as a consequence of designing molecules large enough and with appropriate hydrophobic interactions to effectively bind within the spacious and predominantly hydrophobic active site of the HIV-1 protease. While such features can contribute to high binding affinity, they frequently present challenges for drug development, including poor aqueous solubility (as predicted for DB01721 [1]), variable oral bioavailability, and complex pharmacokinetic behavior. Addressing these challenges often requires sophisticated formulation strategies or the co-administration of pharmacokinetic enhancers, although such considerations are beyond the scope of the experimental data available for DB01721. The predicted low water solubility is a direct consequence of its structure and a significant factor that would need to be addressed if further development were pursued.

E. Chemical Classification

DB01721 is classified based on its structure, origin, and potential use:

• General Type: It is a Small Molecule drug candidate.[1]
• Developmental Stage: It is categorized under Experimental groups in DrugBank, indicating it has not received regulatory approval for therapeutic use.[1]
• Hierarchical Chemical Classification (COCONUT database [8]):
• Super class: Organic acids and derivatives.
• Class: Carboxylic acids and derivatives.
• Sub class: Amino acids, peptides, and analogues.
• Direct parent: Alpha amino acid amides.
• Natural Product (NP) Classification (COCONUT database [8]):
• Pathway: Alkaloids. This is a very broad classification; the compound's structure is more accurately defined as a peptidomimetic.
• Other Classifications (PubChem [7]):
• Drugs: It is listed under "Pharmaceuticals" and is included in databases such as ZINC15 and the NORMAN Suspect List Exchange.

The classification of DB01721 as an "Alpha amino acid amide" and belonging to the subclass of "Amino acids, peptides, and analogues" [8] is particularly informative. This categorization underscores its nature as a peptidomimetic. HIV-1 protease naturally cleaves specific peptide sequences within the viral Gag and Gag-Pol polyproteins.[1] Peptidomimetic inhibitors like DB01721 are designed to mimic these natural substrates, thereby fitting into the enzyme's active site and blocking its function. The presence of amide bonds and structural motifs that resemble amino acid side chains allows these inhibitors to make critical interactions with the various subsites (S1, S2, S1', S2', etc.) of the protease active site. This rational design approach, while effective for achieving potent inhibition, often contributes to the larger molecular size and physicochemical properties (e.g., borderline "drug-likeness") commonly observed in this class of antiviral agents.

III. Pharmacological Profile

A. Mechanism of Action: HIV-1 Protease Inhibition

The primary pharmacological action of DB01721 is the inhibition of the Human Immunodeficiency Virus type 1 (HIV-1) protease.[1] This viral enzyme plays an indispensable role in the HIV-1 replication cycle. HIV-1 protease is responsible for the post-translational cleavage of the viral Gag and Gag-Pol polyprotein precursors into individual, mature structural proteins and essential viral enzymes (such as reverse transcriptase, integrase, and the protease itself).[1] This proteolytic processing is a critical step for the assembly of new, infectious virions.

DB01721, by virtue of its structural design as a peptidomimetic, is believed to act as a competitive inhibitor. It binds to the active site of the HIV-1 protease, thereby preventing the enzyme from processing its natural polyprotein substrates. This blockade of proteolytic activity results in the production of immature, non-infectious viral particles, ultimately suppressing viral replication and reducing viral load in an infected individual.[1]

While DrugBank indicates that the specific mechanism of action for DB01721 is "Not Available" [1], this likely refers to a lack of dedicated, in-depth published studies detailing the finer nuances of its enzyme kinetics or interaction dynamics beyond its established role as an Indinavir analogue and its direct binding to the protease active site. The structural data from PDB entry 1k6v, where DB01721 (as XN2) is co-crystallized within the HIV-1 protease active site [2], along with its design as an analogue of Indinavir (a well-characterized HIV protease inhibitor [6]), provides compelling evidence for its function as a competitive inhibitor of this viral enzyme. For the purposes of this report, its mechanism is confidently defined by this inhibitory action on HIV-1 protease.

B. Primary Biological Target: HIV-1 Gag-Pol Polyprotein (Protease domain)

The direct molecular target of DB01721 is the protease enzyme domain which is encoded within the larger Gag-Pol polyprotein of HIV-1.[1] The Gag-Pol polyprotein (a representative UniProt ID being P35963 [1]) serves as a precursor that must be precisely cleaved by the viral protease to liberate the functional components of the virus.

HIV-1 protease is classified as an aspartic protease (or aspartyl protease). Its catalytic activity relies on a pair of conserved aspartic acid residues (typically Asp25 from each monomer of the homodimeric enzyme) located at the enzyme's active site.[1] These residues are crucial for the hydrolysis of peptide bonds in the viral polyproteins. DB01721 is designed to interact with this active site, thereby obstructing its enzymatic function.

C. Pharmacodynamics (Predicted or Inferred)

Experimentally determined, specific pharmacodynamic parameters for DB01721 are not available in the provided DrugBank summary.[1] However, as an HIV-1 protease inhibitor, its principal pharmacodynamic effect would be the dose-dependent suppression of HIV-1 replication. In a clinical or in vivo setting, this would ideally translate to a measurable reduction in plasma HIV RNA levels (viral load).

The potency of this antiviral effect is typically quantified by parameters such as the half-maximal inhibitory concentration (IC50​) or half-maximal effective concentration (EC50​) against viral replication in cell-based assays, or by the inhibition constant (Ki​) in cell-free enzymatic assays against purified protease. For DB01721 (investigated under the research code XN2 or XN1336-52), IC50​ values against purified HIV-1 protease (both wild-type and a multi-drug resistant variant) and dissociation constants (Kd​) have been reported in the study by Schiffer C.A. et al. (2002).[4] These specific activity values are detailed in Section IV of this report.

D. Pharmacokinetics (Predicted ADMET Features and Inferred Properties)

No experimental pharmacokinetic data for DB01721 are available from the provided research snippets. However, DrugBank [1] furnishes a comprehensive suite of in silico predicted ADMET (Absorption, Distribution, Metabolism, Excretion, Toxicity) properties, which offer preliminary insights into its potential disposition and safety profile:

Table 2: Predicted ADMET Properties of DB01721 (from DrugBank [1])

ADMET Parameter	Predicted Value/Classification	Probability (if available)
Absorption & Distribution
Water Solubility	0.0233 mg/mL (Low)
LogP (Octanol/Water Partition Coeff.)	4.37 / 3.94
Human Intestinal Absorption (HIA)	0.5756 (Moderate to Good)
Blood Brain Barrier (BBB) Permeability	0.9872 (Predicted to Cross)
Caco-2 Permeability	0.7597 (Permeable)
Metabolism & Excretion
P-glycoprotein (P-gp) Substrate	Yes	0.9193
P-gp Inhibitor I	Yes	0.5826
P-gp Inhibitor II	No	0.8226
CYP2C9 Inhibitor	Yes (Predicted)
CYP2D6 Inhibitor	Yes (Predicted)
CYP3A4 Inhibitor	Yes (Predicted)
CYP3A4 Substrate	Yes (Predicted)
CYP450 Inhibitory Promiscuity	Low	0.8294
Renal Organic Cation Transporter (OCT2)	Non-inhibitor	0.8833
Biodegradation	Not readily biodegradable	0.9718
Toxicity
Ames Test	Non-AMES toxic	0.715
Carcinogenicity	Non-carcinogen	0.8739
Rat Acute Toxicity (LD50, mol/kg)	2.5504
hERG Inhibition (Predictor I)	Weak inhibitor	0.9684
hERG Inhibition (Predictor II)	Inhibitor	0.5234

These predicted ADMET properties, while computational, provide an initial framework for assessing the potential pharmacokinetic behavior and safety liabilities of DB01721. For instance, the predicted status as a P-glycoprotein substrate and inhibitor, along with the predicted inhibition of key cytochrome P450 enzymes (notably CYP3A4), flags a high potential for drug-drug interactions. This is a common characteristic of HIV protease inhibitors; Indinavir itself is a known CYP3A4 substrate and inhibitor, leading to a complex interaction profile with co-administered medications.[14] If DB01721 were to share this profile, its clinical use would necessitate careful management of concomitant therapies. Furthermore, its status as a P-gp substrate could influence its intracellular accumulation in target cells and its passage across biological barriers, potentially impacting its efficacy and distribution.

The prediction of hERG (human Ether-à-go-go-Related Gene) potassium channel inhibition, even if characterized as "weak" by one predictor model [1], constitutes a significant safety concern. hERG channel blockade is mechanistically linked to QT interval prolongation on the electrocardiogram, which can increase the risk of potentially fatal cardiac arrhythmias such as Torsades de Pointes. This is a common cause of drug candidate failure during preclinical or clinical development, and any hERG liability for DB01721 would require thorough experimental investigation. These predictive data collectively highlight that while an analogue might be designed for improved target potency, its overall pharmacokinetic and safety profile remains a critical determinant of its ultimate therapeutic viability.

IV. Biological Activity and Investigational Research

A. Context as an Indinavir Analogue

DB01721, also identified by research codes XN2 and XN1336-52, was developed and investigated as a structural analogue of Indinavir.[1] Indinavir (Crixivan®) is a potent peptidomimetic inhibitor of HIV-1 protease that was approved for the treatment of HIV/AIDS.[6] However, its clinical utility has been hampered by factors including the development of viral resistance, a demanding dosing schedule requiring significant water intake to prevent nephrolithiasis, and various metabolic side effects.[6] The rationale behind developing Indinavir analogues, such as DB01721, is to address these limitations by creating new chemical entities with potentially improved characteristics. These improvements could include enhanced potency against drug-resistant HIV-1 strains, more favorable pharmacokinetic properties (e.g., increased metabolic stability, better oral bioavailability), or a reduced toxicity profile.[5] The research involving DB01721 specifically focused on its interaction with a multi-drug resistant form of HIV-1 protease.

B. Focus: Inhibition of Multi-Drug Resistant HIV-1 Protease – Detailed Analysis of PDB Entry 1k6v and Schiffer C.A. et al. (2002)

The most definitive research characterizing DB01721 is documented in the study by King N.M., Schiffer C.A., and colleagues, published in Protein Science in 2002, and associated with the Protein Data Bank (PDB) entry 1k6v.[3] This work provides a detailed structural and biochemical analysis of DB01721 (referred to as XN2 in the PDB and XN1336-52 in the paper) in complex with a multi-drug-resistant (MDR) variant of HIV-1 protease.

• PDB Entry 1k6v Details:
• Classification: Hydrolase (HIV-1 Protease).[2]
• Source Organism: Human immunodeficiency virus 1.[2]
• Expression System for Protease: Escherichia coli.[2]
• Protease Mutations: The study utilized an MDR HIV-1 protease variant containing three key resistance-associated mutations: L63P, V82T, and I84V (referred to as "3X-protease" or "MN-variant" in the publication).[2]
• Experimental Method: X-ray Diffraction.[2]
• Resolution: 2.00 Å.[2]
• Bound Ligand: XN2 (which is DB01721 / XN1336-52).[2]
• Structural Modifications of DB01721 (XN2/XN1336-52) Compared to Indinavir: DB01721 (XN1336-52) was designed by incorporating two distinct structural modifications onto the Indinavir scaffold, these modifications having been previously explored in other singly modified analogues 4:

1. P1 Site Modification: An S-configured methyl group was added to the benzylic carbon of Indinavir's P1 phenylmethyl side chain, resulting in a (1-phenylethyl) moiety. This modification was also present in a singly modified analogue, XN1336-51.
2. P2' Site Modification: The P2' pyridine ring of Indinavir was replaced with a methylenedioxyphenyl group (specifically, a benzodioxol-5-ylmethyl group). This modification was present in another singly modified analogue, 807-29-4.

• Binding Affinity and Activity Data (from Schiffer et al., 2002 4): The study by Schiffer et al. meticulously evaluated the inhibitory potency (IC50​) and binding affinity (dissociation constant, Kd​) of Indinavir and its analogues, including DB01721 (XN1336-52), against both wild-type (WT) HIV-1 protease and the 3X-resistant (L63P/V82T/I84V) protease variant. Table 3: Comparative Binding Affinities and Inhibitory Potencies of Indinavir and Analogues against HIV-1 Protease Variants (Data from Schiffer et al., 2002 [4])

Compound	Target Protease	IC50​ (µM)	Kd​ (M)
Indinavir	Wild-Type	Not explicitly stated in table format for IC50, but generally highly potent	8.6×10−10
Indinavir	3X-Resistant (L63P/V82T/I84V)	0.47	3.3×10−8
XN1336-51 (P1 S-methylbenzyl)	Wild-Type	Not explicitly stated in table format for IC50	1.2×10−9
XN1336-51 (P1 S-methylbenzyl)	3X-Resistant (L63P/V82T/I84V)	0.40	2.4×10−8
807-29-4 (P2' methylenedioxyphenyl)	Wild-Type	Not explicitly stated in table format for IC50	5.3×10−10
807-29-4 (P2' methylenedioxyphenyl)	3X-Resistant (L63P/V82T/I84V)	0.31	4.4×10−8
DB01721 (XN2/XN1336-52; P1+P2' mods)	Wild-Type	Not explicitly stated in table format for IC50	1.5×10−9
DB01721 (XN2/XN1336-52; P1+P2' mods)	3X-Resistant (L63P/V82T/I84V)	1.2	1.2×10−7

The data clearly demonstrate that while the singly modified analogues (XN1336-51 and 807-29-4) maintained or slightly improved potency against the 3X-resistant protease compared to Indinavir, the dually modified DB01721 (XN1336-52) exhibited significantly *reduced* potency. Its <span class="math-inline">IC\_\{50\}</span> against the 3X-resistant protease was 1.2 µM, which is 2.5 to 3.8 times weaker than Indinavir and the singly modified compounds. Similarly, its binding affinity (<span class="math-inline">K\_d</span>) for the 3X-resistant protease was also notably poorer. Against the wild-type protease, DB01721 (<span class="math-inline">K\_d \= 1\.5 \\times 10^\{\-9\}</span> M) was less potent than Indinavir (<span class="math-inline">K\_d \= 8\.6 \\times 10^\{\-10\}</span> M) and the P2'-modified analogue 807-29-4 (<span class="math-inline">K\_d \= 5\.3 \\times 10^\{\-10\}</span> M).

• Observed Structural Interactions and Implications for Drug Design (from Schiffer et al., 2002 4): The central finding of the Schiffer et al. study was a "lack of synergy" when the two individually tolerated or beneficial modifications were combined in DB01721. The structural analysis of the protease-inhibitor complexes (PDB entry 1k6v for XN2) revealed that the conformational adaptations within the protease active site that favorably accommodated one modification were often mutually exclusive with those required for the other. For instance, the P2' methylenedioxyphenyl group in analogue 807-29-4 induced certain beneficial structural rearrangements in the S1/S3 subsites of the protease, particularly around residue Pro81 of the second monomer (P81b). However, when this modification was combined with the P1 S-methylbenzyl group in DB01721, the protease was unable to adopt these same advantageous conformations simultaneously. The crystal structure of the DB01721-protease complex was found to be more similar to that of the XN1336-51 (P1 S-methylbenzyl only) complex, suggesting that the P1 modification had a more dominant, and in this combined context, detrimental influence on the overall binding mode. This outcome serves as a critical lesson in structure-based drug design: the assumption that multiple, individually positive modifications will yield additive or synergistic benefits is not always valid. The protein target's conformational flexibility and its ability to adapt to different ligand features play a crucial role. In the case of DB01721, the combined steric and electronic demands of the dual modifications appeared to exceed the protease's capacity for favorable induced fit, leading to compromised binding. This highlights the intricate interplay of protein-ligand interactions and the challenge of prospectively designing highly potent inhibitors, especially against adaptable targets like viral enzymes prone to resistance mutations. The study underscores that a deep understanding of how the target protein structurally responds to individual and combined ligand modifications is essential for advancing inhibitor design.

C. Synthesis of DB01721 (XN2 / XN1336-52)

The synthesis of DB01721 (XN1336-52) was reported by Schiffer et al. (2002).[4] The key steps involved:

1. Preparation of a Modified Precursor (Compound I): The synthesis started from a penultimate piperazine intermediate, referred to as "compound I" in the paper's scheme. To incorporate the P1 S-methylbenzyl modification, (S)-3-phenylbutyric acid (purchased from Fluka) was used in place of hydrocinnamic acid (which would yield the standard Indinavir P1 benzyl group) during the formation of this piperazine precursor.
2. Reductive Alkylation: The modified compound I, now containing the desired P1 (1-phenylethyl) side chain, was then subjected to reductive alkylation with piperonal (3,4-methylenedioxybenzaldehyde). Piperonal provides the methylenedioxyphenyl moiety for the P2' position. This reaction was carried out in the presence of sodium triacetoxyborohydride (NaBH(OAc)3​) as the reducing agent.
3. Purification: The final product, DB01721 (XN1336-52), was purified using preparative thin-layer chromatography (TLC) with a solvent system of 10% methanol in methylene chloride.
4. Characterization: The structure and purity of the synthesized compound were confirmed by proton (1H) and carbon (13C) Nuclear Magnetic Resonance (NMR) spectroscopy and mass spectrometry, with no evidence of hydration reported.

The syntheses of the singly modified analogues XN1336-51 (P1 S-methylbenzyl only) and 807-29-4 (P2' methylenedioxyphenyl only) followed analogous synthetic strategies, utilizing the appropriate aldehyde (pyridine-3-carboxaldehyde for XN1336-51, and piperonal for 807-29-4 with the original Indinavir P1 precursor) in the reductive alkylation step.[4]

D. Related Research on Indinavir Analogues

The development of DB01721 occurred within a broader context of research aimed at optimizing Indinavir and other HIV protease inhibitors. Several distinct strategies have been pursued:

• Improving Pharmacokinetics and Potency by Blocking Metabolism: Research by Yuan Cheng and colleagues at Merck focused on designing Indinavir analogues where known sites of metabolic degradation were blocked.[21] This approach aimed to improve the pharmacokinetic profile (e.g., half-life, oral bioavailability) of the inhibitors. One notable series from this work involved cis-aminochromanol substituted analogues, which reportedly exhibited excellent potency against both wild-type HIV (NL4-3 strain) and protease inhibitor-resistant HIV strains, alongside improved pharmacokinetic profiles in animal models.[21] This strategy of enhancing metabolic stability is complementary to the approach of modifying active site interactions (as in DB01721), both being vital for developing clinically viable drugs.
• Solid-Phase Synthesis and Combinatorial Libraries: To accelerate the discovery of novel Indinavir analogues, solid-phase synthesis techniques and combinatorial chemistry have been employed.[19] These methods allow for the rapid generation of diverse libraries of related compounds, which can then be screened in high-throughput assays to identify leads with enhanced potency against HIV protease (including resistant variants) or more favorable pharmacokinetic properties.
• Exploration of Alternative Biological Activities: Beyond antiviral activity, some Indinavir analogues have been investigated for other therapeutic potentials. For example, the analogue CH05-10 was reported to possess broad-spectrum antitumor activity by inducing cell cycle arrest, apoptosis, endoplasmic reticulum stress, and autophagy in cancer cells.[20] While this is distinct from the anti-HIV focus of DB01721, it illustrates the potential for chemical scaffolds developed for one target to exhibit activity in other biological contexts.
• Patent Landscape: Indinavir itself was patented by Merck & Co. in 1991.[6] The field of HIV protease inhibitors is characterized by extensive patenting activity. Numerous patents cover various classes of inhibitors, including those with indoleoxoacetyl piperazine derivatives [26], and specific processes for synthesizing Indinavir, its intermediates, or related analogues.[17] Some patents specifically describe HIV protease inhibitors incorporating a methylenedioxyphenyl moiety [29], a structural feature also present in DB01721. The precursor for the analogue 807-29-4 (which shares the P2' methylenedioxyphenyl group with DB01721) was reportedly prepared according to a Merck patent, likely US 5,413,999A, which covers Indinavir and related compounds.[4]

These varied research efforts underscore the multifaceted approaches taken to refine and build upon the initial success of HIV protease inhibitors like Indinavir, addressing challenges such as resistance, pharmacokinetics, and exploring new therapeutic applications.

V. Preclinical and Clinical Development Status

A. Current Investigational Status

DB01721 is classified as an experimental compound.[1] The primary and most detailed characterization of this molecule in the provided research materials dates back to the 2002 publication by Schiffer C.A. et al..[4] There is no indication from the available information that DB01721 has progressed into formal clinical development or received any regulatory approvals.

B. Summary of Available Data on:

• Therapeutic Indications (Anticipated): Given its design as an HIV-1 protease inhibitor and its investigation against drug-resistant viral strains, the anticipated therapeutic indication for DB01721 would be the treatment of HIV-1 infection. The focus on multi-drug resistant protease suggests a potential role in salvage therapy regimens. However, the reported reduced potency of DB01721 against the tested MDR strain compared to Indinavir and its singly modified counterparts [4] makes its own progression as a therapeutic agent in its current form unlikely without significant further optimization.
• Adverse Effects and Toxicity (Predicted or Inferred):
• Specific experimental adverse effect data for DB01721 are listed as "Not Available" in DrugBank.[1]
• Predicted Toxicity (from DrugBank [1]):
• Genotoxicity: Predicted to be Non-AMES toxic.
• Carcinogenicity: Predicted to be a Non-carcinogen.
• Acute Systemic Toxicity: Predicted Rat acute LD50 of 2.5504 mol/kg.
• Cardiotoxicity: Potential for hERG inhibition is predicted (Weak inhibitor by one model, Inhibitor by another), suggesting a possible risk for cardiac arrhythmias that would require experimental evaluation.
• Inferred Class-Specific Effects: As an analogue of Indinavir, DB01721 might be anticipated to share some of the class-specific adverse effects associated with HIV protease inhibitors. These can include gastrointestinal disturbances (nausea, diarrhea), metabolic alterations (hyperlipidemia, hyperglycemia, insulin resistance, lipodystrophy), and, particularly for Indinavir, nephrolithiasis (kidney stones) due to its low aqueous solubility and crystallization in the urinary tract.[6] However, without specific data for DB01721, these remain speculative.
• Drug Interactions (Predicted):
• Based on in silico predictions from DrugBank [1], DB01721 has a high potential for drug-drug interactions. It is predicted to be a substrate and inhibitor of P-glycoprotein (P-gp) and an inhibitor of several cytochrome P450 (CYP) enzymes, including CYP3A4, CYP2C9, and CYP2D6.
• The predicted interaction with CYP3A4 is particularly noteworthy, as this enzyme is responsible for the metabolism of many drugs, including most HIV protease inhibitors like Indinavir.[14] Inhibition of CYP3A4 by DB01721 could lead to increased concentrations and potential toxicity of co-administered CYP3A4 substrates. Conversely, if DB01721 is also a CYP3A4 substrate, its own concentrations could be affected by CYP3A4 inducers or inhibitors.

C. Clinical Trial Information

There are no clinical trials listed for DB01721 in Phases 0, 1, 2, 3, or 4 in the DrugBank database.[1] Searches using other identifiers such as PD060269 or PubChem CID 5496642 in clinical trial registries did not yield any specific trials for this compound.[32]

The absence of clinical trial data, combined with the 2002 publication date of its primary characterization which demonstrated that the dual modifications in DB01721 led to reduced potency against the MDR HIV-1 protease [4], strongly suggests that this specific compound did not advance into clinical development. It is more likely that DB01721 remained a research tool or a case study that contributed to the understanding of inhibitor design principles rather than progressing as a viable drug candidate itself. Compounds exhibiting a less favorable preclinical profile, particularly in terms of target potency compared to existing agents or simpler analogues, are typically not selected for the resource-intensive process of clinical evaluation.

VI. Other Relevant Information

A. Physical State

The predicted physical state of DB01721 at standard conditions is solid, according to DrugBank.[1]

B. Patent Information

DrugBank lists zero patents specifically for DB01721.1 However, the field of Indinavir and its analogues is extensively covered by patents, primarily assigned to Merck & Co., the original developer of Indinavir. Indinavir itself was patented in 1991.6

The synthesis of the P2' methylenedioxyphenyl intermediate, which is a structural component of DB01721 (via analogue 807-29-4), was based on a Merck patent (US 5,413,999A, as cited in the context of Indinavir preparation 17) according to the Schiffer et al. (2002) paper.4 This patent covers Indinavir and related HIV protease inhibitors. Therefore, while DB01721 as a distinct entity may not be individually patented or may have been covered under broader Markush claims of existing patents, its structural motifs and the general class of Indinavir analogues fall within a well-established patent landscape for HIV protease inhibitors.26

C. Chemical Suppliers

Specific information regarding commercial chemical suppliers for DB01721 (PubChem CID 5496642) is not explicitly detailed in the provided research snippets. General chemical supplier advertisements, such as BOC Sciences and TCI America, were noted in one snippet [37], but these were in the context of a different chemical compound (5-hydroxypentan-2-one) and do not confirm availability of DB01721. Given its status as an experimental compound primarily characterized in a 2002 research paper, DB01721 is unlikely to be a readily available catalog chemical. Its acquisition would most likely require custom synthesis. Generic supplier information for peptides or other chemicals [38] is not directly applicable.

D. BioAssay Data from PubChem

The PubChem entry for CID 5496642 (DB01721) includes sections for "Biological Test Results" and "Interactions and Pathways".[7] A general search for bioactivity related to synonyms of DB01721 (PD060269, XN2, 1k6v) and HIV yielded some contextual results.[10] One result referred to the general challenges in HIV protease inhibitor design and the promiscuity of early aspartyl protease inhibitors.[40] Another result pertained to Xanthohumol (abbreviated XN), which is a distinct compound and not the XN2 ligand relevant here, highlighting a potential point of confusion if identifiers are not carefully managed.[41]

The most relevant bioactivity information directly linked to DB01721 (CID 5496642) in PubChem is its association as the ligand XN2 in the PDB entry 1k6v.[7] This PDB entry, and the associated publication by Schiffer et al. (2002) [3], remains the primary source of detailed public bioactivity data (i.e., IC50​ and Kd​ values against HIV-1 protease) for this compound. Beyond this specific structural and biochemical study, there is no indication in the provided materials of extensive public bioassay screening data for DB01721. Its characterization appears largely confined to this focused research project aimed at understanding structure-activity relationships in the context of drug resistance.

VII. Conclusion and Future Perspectives

A. Summary of Key Findings for DB01721 (XN2 / PD060269 / XN1336-52)

DB01721, systematically named N-[2-hydroxy-1-indanyl]-5-[(2-tertiarybutylaminocarbonyl)-4(benzodioxol-5-ylmethyl)-piperazino]-4-hydroxy-2-(1-phenylethyl)-pentanamide, is an experimental analogue of the HIV protease inhibitor Indinavir. Its design incorporated two specific structural modifications to the Indinavir scaffold: an S-methyl group at the P1 benzylic position and a methylenedioxyphenyl moiety replacing the P2' pyridine ring.

The principal investigation of DB01721 focused on its efficacy as an inhibitor of HIV-1 protease, with particular emphasis on its activity against a multi-drug resistant variant (L63P/V82T/I84V). The seminal study by Schiffer C.A. et al. (2002), associated with PDB entry 1k6v, revealed that, contrary to the potential expectation of synergistic or additive benefits from combining two individually tolerated modifications, DB01721 exhibited reduced binding affinity and inhibitory potency against both wild-type and the 3X-resistant HIV-1 protease when compared to Indinavir and the corresponding singly modified analogues.[4] Structural analysis from the co-crystal structure (PDB: 1k6v) indicated that the protease active site could not optimally accommodate both modifications simultaneously, leading to this observed lack of synergy.

B. Significance in the Context of HIV-1 Protease Inhibitor Research

The study of DB01721 provides a significant case study in the field of structure-based drug design, particularly for HIV-1 protease inhibitors. It underscores the principle that combining structural modifications, even those that individually appear neutral or beneficial, does not invariably lead to an improved compound. The outcome for DB01721 highlights the intricate and often unpredictable nature of protein-ligand interactions, the critical role of protein conformational flexibility and adaptability ("induced fit"), and the formidable challenges posed by viral drug resistance.

This research emphasizes that a nuanced understanding of how multiple modifications interact—both sterically and allosterically—to affect the overall binding free energy is paramount. The "negative data" generated from the DB01721 investigation, i.e., the documented failure to achieve enhanced potency through these combined modifications, is scientifically valuable. It informs the field about specific structural combinations that are counterproductive for this particular scaffold and target, thereby guiding future design efforts away from similar pitfalls and towards more promising strategies. Such findings refine the SAR models for this class of inhibitors and contribute to a more sophisticated approach to overcoming drug resistance.

C. Potential and Challenges in Further Development

Given the reported reduced potency in the key published study [4], DB01721 itself is not a strong candidate for further clinical development in its current form. Significant redesign would be necessary to overcome the observed detrimental effects of the combined modifications.

However, the insights gleaned from its detailed structural and biochemical characterization remain valuable for the broader endeavor of HIV protease inhibitor design. Understanding precisely why this specific combination of P1 and P2' modifications failed to enhance (and in fact, diminished) activity can inform the development of novel scaffolds or different combinations of substituents that might better exploit the adaptive capacity of the protease active site. The overarching challenge in HIV therapeutics persists: to design protease inhibitors that are not only highly potent against wild-type virus but also maintain robust activity against a wide spectrum of clinically relevant resistant variants, while simultaneously possessing favorable pharmacokinetic profiles (good oral bioavailability, manageable drug interactions, appropriate half-life) and minimal toxicity. The story of DB01721 contributes a crucial, albeit cautionary, chapter to this ongoing scientific pursuit.

Works cited

1. 5-[(2-tertiarybutylaminocarbonyl)-4(benzo[1,3]dioxol-5 ... - DrugBank, accessed June 4, 2025, https://go.drugbank.com/drugs/DB01721
2. 1K6V: LACK OF SYNERGY FOR INHIBITORS TARGETING A MULTI-DRUG RESISTANT HIV-1 PROTEASE - RCSB PDB, accessed June 4, 2025, https://www.rcsb.org/structure/1k6v
3. 1K6V: LACK OF SYNERGY FOR INHIBITORS ... - RCSB PDB, accessed June 4, 2025, https://www.rcsb.org/structure/1K6V
4. Lack of synergy for inhibitors targeting a multi-drug-resistant HIV-1 ..., accessed June 4, 2025, https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2373441/
5. Lack of synergy for inhibitors targeting a multi-drug-resistant HIV-1 protease - PMC, accessed June 4, 2025, https://pmc.ncbi.nlm.nih.gov/articles/PMC2373441/
6. Indinavir - Wikipedia, accessed June 4, 2025, https://en.wikipedia.org/wiki/Indinavir
7. N-(2-hydroxy-1-indanyl)-5-((2-tertiarybutylaminocarbonyl)-4(benzo(1,3)dioxol-5-ylmethyl)-piperazino) - PubChem, accessed June 4, 2025, https://pubchem.ncbi.nlm.nih.gov/compound/5496642
8. cnp0514278.1: n-[2-hydroxy-1-indanyl]-5-[(2-tertiarybutylaminocarbonyl)-4(benzo[1,3]dioxol-5-ylmethyl)-piperazino] - COCONUT Natural Products, accessed June 4, 2025, https://coconut.naturalproducts.net/compound/coconut_id/CNP0514278.1
9. Full wwPDB X-ray Structure Validation Report i - BMRB, accessed June 4, 2025, https://bmrb.protein.osaka-u.ac.jp/ftp/pub/pdb/validation_reports/k6/1k6v/1k6v_full_validation.pdf.gz
10. N-(2-hydroxy-1-indanyl)-5-((2 ... - PubChem, accessed June 4, 2025, https://pubchem.ncbi.nlm.nih.gov/compound/5496642#section=BioAssay-Results
11. PD060269 (PD060269, MJIRDPUZGGHJMX-OIVSQUILSA-N), accessed June 4, 2025, https://www.probes-drugs.org/compound/PD060269/
12. www.ebi.ac.uk, accessed June 4, 2025, https://www.ebi.ac.uk/chembl/compound_report_card/CHEMBL1257519/
13. DB code: D00471 - EzCatDB, accessed June 4, 2025, https://ezcatdb.cbrc.pj.aist.go.jp/EzCatDB/enzyme/D00471
14. Indinavir | C36H47N5O4 | CID 5362440 - PubChem, accessed June 4, 2025, https://pubchem.ncbi.nlm.nih.gov/compound/Indinavir
15. HIV protease inhibitors: a review of molecular selectivity and toxicity - Dove Medical Press, accessed June 4, 2025, https://www.dovepress.com/hiv-protease-inhibitors-a-review-of-molecular-selectivity-and-toxicity-peer-reviewed-fulltext-article-HIV
16. Indinavir: Uses, Interactions, Mechanism of Action | DrugBank Online, accessed June 4, 2025, https://go.drugbank.com/drugs/DB00224
17. Indinavir, accessed June 4, 2025, https://www.drugfuture.com/chemdata/indinavir.html
18. HIV protease inhibitors: a review of molecular selectivity and toxicity - PubMed Central, accessed June 4, 2025, https://pmc.ncbi.nlm.nih.gov/articles/PMC4396582/
19. Solid Phase Synthesis of Indinavir and Its Analogues | Request PDF - ResearchGate, accessed June 4, 2025, https://www.researchgate.net/publication/12295541_Solid_Phase_Synthesis_of_Indinavir_and_Its_Analogues
20. CH05-10, a novel indinavir analog, is a broad-spectrum antitumor agent that induces cell cycle arrest, apoptosis, endoplasmic reticulum stress and autophagy - PubMed, accessed June 4, 2025, https://pubmed.ncbi.nlm.nih.gov/20946116/
21. Indinavir analogues with blocked metabolism sites as HIV protease ..., accessed June 4, 2025, https://pubmed.ncbi.nlm.nih.gov/12161147/
22. Indinavir Analogues with Blocked Metabolism Sites as HIV Protease ..., accessed June 4, 2025, https://www.researchgate.net/publication/11225760_Indinavir_Analogues_with_Blocked_Metabolism_Sites_as_HIV_Protease_Inhibitors_with_Improved_Pharmacological_Profiles_and_High_Potency_Against_PI-Resistant_Viral_Strains
23. Design and synthesis of highly potent HIV protease inhibitors with activity against resistant virus | Request PDF - ResearchGate, accessed June 4, 2025, https://www.researchgate.net/publication/10774956_Design_and_synthesis_of_highly_potent_HIV_protease_inhibitors_with_activity_against_resistant_virus
24. Molecular dynamics simulations of 14 HIV protease mutants in complexes with indinavir, accessed June 4, 2025, https://ouci.dntb.gov.ua/en/works/4LjBj1K9/
25. 1K6C: LACK OF SYNERGY FOR INHIBITORS TARGETING A MULTI-DRUG RESISTANT HIV-1 PROTEASE - RCSB PDB, accessed June 4, 2025, https://www.rcsb.org/structure/1k6c
26. US6469006B1 - Antiviral indoleoxoacetyl piperazine derivatives - Google Patents, accessed June 4, 2025, https://patents.google.com/patent/US6469006B1/en
27. US5981759A - Process for indinavir intermediate - Google Patents, accessed June 4, 2025, https://patents.google.com/patent/US5981759A/en
28. Process for making HIV protease inhibitors - Justia Patents, accessed June 4, 2025, https://patents.justia.com/patent/5693803
29. WO2004014380A1 - Composition and antiviral activity of substituted azaindoleoxoacetic piperazine derivatives - Google Patents, accessed June 4, 2025, https://patents.google.com/patent/WO2004014380A1/en
30. SK9592003A3 - Composition and antiviral activity of substituted azaindoleoxoacetic piperazine derivatives - Google Patents, accessed June 4, 2025, https://patents.google.com/patent/SK9592003A3/en
31. US5484926A - HIV protease inhibitors - Google Patents, accessed June 4, 2025, https://patents.google.com/patent/US5484926A/en
32. Efficacy and Safety of Subcutaneous Dupilumab in Participants With Asthma/Asthmatic Wheeze Aged 2 to <6 Years (LIBERTY ASTHMA TREKIDS) - Clinical Trials | Regeneron Pharmaceuticals, accessed June 4, 2025, https://clinicaltrials.regeneron.com/clinical-trials/a0Mdy0000001BMPEA2/nct06191315
33. A Study to Investigate the Effectiveness of Tirzepatide (LY3298176) Following Initiation of Ixekizumab (LY2439821) in Participants With Moderate-to-Severe Plaque PsO and Obesity or Overweight in Clinical Practice (TOGETHER AMPLIFY-PsO) - Lilly Trials, accessed June 4, 2025, https://trials.lilly.com/en-US/trial/580055
34. Clinical Trials Finder - CFF.org, accessed June 4, 2025, https://apps.cff.org/Trials/Finder?search=%7BPipelineDrugId%3A%2710166%27%2CPipelineDrugDisplayName%3A%27Vanzacaftor+%2B+tezacaftor+%2B+deutivacaftor+(Alyftrek)%27%7D
35. Efficacy study of GSK's investigational respiratory syncytial virus (RSV) vaccine in adults aged 60 years and above, accessed June 4, 2025, https://www.gsk-studyregister.com/en/trial-details/?id=212494
36. Synthesis of Indane and its analogues.[4,5,11] - ResearchGate, accessed June 4, 2025, https://www.researchgate.net/figure/Synthesis-of-Indane-and-its-analogues4-5-11_fig1_350126246
37. 5-hydroxypentan-2-one, 1071-73-4, accessed June 4, 2025, http://www.thegoodscentscompany.com/data/rw1698311.html
38. Peptides - Amino Acids and Derivatives - Spectrum Chemical, accessed June 4, 2025, https://www.spectrumchemical.com/chemical/peptides
39. Benzyl alcohol | 108006 - Honeywell Research Chemicals, accessed June 4, 2025, https://lab.honeywell.com/shop/benzyl-alcohol-108006
40. Novel method for probing the specificity binding profile of ligands: applications to HIV protease. - SciSpace, accessed June 4, 2025, https://scispace.com/pdf/novel-method-for-probing-the-specificity-binding-profile-of-285c5n7nbk.pdf
41. Xanthohumol, a novel anti-HIV-1 agent purified from Hops - ResearchGate, accessed June 4, 2025, https://www.researchgate.net/publication/8177560_Xanthohumol_a_novel_anti-HIV-1_agent_purified_from_Hops