Aminohippuric Acid (DB00345): A Comprehensive Pharmacological and Clinical Monograph

Executive Summary

Aminohippuric acid, also known as para-aminohippuric acid (PAH), is a small molecule diagnostic agent that has served for decades as the clinical and physiological gold standard for the measurement of renal hemodynamics.[1] As a synthetic derivative of hippuric acid, formed from the amino acid glycine and para-aminobenzoic acid, it is not found endogenously in humans and must be administered intravenously for its diagnostic purposes.[1] The foundational principle of its utility lies in its unique and remarkably efficient handling by the kidneys. In a healthy individual, aminohippuric acid is almost completely extracted from the blood in a single transit through the renal circulation, a process achieved through a dual mechanism of glomerular filtration and highly active tubular secretion.[3]

This high renal extraction ratio, approximately 92%, allows its plasma clearance to be used as a precise and reliable measure of Effective Renal Plasma Flow (ERPF).[1] Furthermore, by infusing the drug to concentrations that saturate the renal transport machinery, it is possible to determine the maximal functional capacity of the tubular secretory mechanism (TmPAH), providing a quantitative index of functional proximal tubular mass.[3] Historically, its properties as a substrate for renal transporters were leveraged in a therapeutic context; during World War II, it was co-administered with penicillin to competitively inhibit the antibiotic's renal clearance, thereby prolonging its therapeutic effect.[1]

Aminohippuric acid is generally considered to have a favorable safety profile when administered under controlled clinical conditions. The primary risks are not related to direct organ toxicity but rather to the potential for hypersensitivity reactions, which can range from mild urticaria to severe anaphylaxis, and to the hemodynamic stress of the intravenous fluid load in patients with compromised cardiac function.[7] Its extensive interaction profile is almost entirely predictable based on its reliance on the organic anion transporter (OAT) system for secretion, leading to competitive interactions with a wide array of anionic drugs. Despite its approval dating back to 1944, aminohippuric acid remains an indispensable tool in nephrology, renal physiology research, and modern clinical trials investigating the cardiorenal effects of novel therapeutics.

Chemical Identity and Physicochemical Properties

The precise identification and characterization of a chemical entity are fundamental to understanding its pharmacological behavior. Aminohippuric acid is well-defined across numerous chemical and biomedical databases.

Nomenclature and Identifiers

Aminohippuric acid is known by several common and systematic names. It is most frequently referred to as para-aminohippuric acid, often abbreviated as PAH or PAHA.[1] Its systematic International Union of Pure and Applied Chemistry (IUPAC) name is (4-Aminobenzamido)acetic acid or, alternatively, 2-[(4-aminobenzoyl)amino]acetic acid.[1] The compound is registered under CAS Number 61-78-9 for the free acid and 94-16-6 for its clinically utilized sodium salt.[1] Its identity is further cataloged by a host of identifiers, including DrugBank ID DB00345, PubChem Compound ID (CID) 2148, and the FDA Unique Ingredient Identifier (UNII) Y79XT83BJ9.[1]

Molecular Structure and Formulation

The molecular formula of aminohippuric acid is C9​H10​N2​O3​, corresponding to a molecular weight of approximately 194.19 g/mol.[1] The sodium salt has the formula

C9​H9​N2​NaO3​ and a molecular weight of approximately 216.18 g/mol.[4] Its chemical structure is defined by a glycine molecule in which the amino group is acylated by a 4-aminobenzoyl group. This structure is unambiguously represented by standard chemical notations such as the Simplified Molecular Input Line Entry System (SMILES) string

C1=CC(=CC=C1C(=O)NCC(=O)O)N and the International Chemical Identifier (InChI) InChI=1S/C9H10N2O3/c10-7-3-1-6(2-4-7)9(14)11-5-8(12)13/h1-4H,5,10H2,(H,11,14)(H,12,13).[9]

For clinical use, aminohippuric acid is provided as its sodium salt, aminohippurate sodium. The pharmaceutical product is a sterile, non-preserved 20% aqueous solution intended for intravenous injection. A typical formulation consists of 2 g of aminohippurate sodium per 10 mL of solution, with sodium hydroxide added as needed to adjust the final pH to a physiologically compatible range of 6.7 to 7.6.[4]

Physicochemical Characteristics

Aminohippuric acid presents as a solid, described as a white to light yellow or even light orange crystalline powder.[16] The free acid exhibits poor solubility in cold water, ether, and carbon tetrachloride, but is soluble in organic solvents such as alcohol, chloroform, benzene, and acetone.[10] In contrast, the sodium salt is readily soluble in water (50 mg/mL), a property that is essential for its preparation as a concentrated aqueous solution for intravenous infusion.[4] The molecule is also described as lipid-insoluble.[4] The melting point of the solid is in the range of 197–202 °C, at which point it undergoes decomposition.[16]

The compound is a weak acid, with a reported pKa of its conjugate acid between 3.61 and 3.83.[4] This indicates that at physiological pH (~7.4), the carboxylic acid group will be deprotonated, and the molecule will exist as an anion, a key factor in its interaction with renal anion transporters. In terms of stability, the compound is known to be sensitive to both light and air.[16] It should be stored at room temperature and protected from light to maintain its integrity.[14] It is chemically incompatible with strong oxidizing agents.[17]

The physicochemical properties of aminohippuric acid are exceptionally well-suited for its function as an intravenous diagnostic agent. The high water solubility of its sodium salt is a prerequisite for creating the concentrated sterile solutions required for clinical infusions. Concurrently, its lipid-insoluble nature is a critical determinant of its pharmacokinetic behavior. By being largely unable to cross cell membranes via passive diffusion, the molecule is confined primarily to the extracellular fluid and plasma compartments following administration. This prevents extensive sequestration into tissues, ensuring that the drug is rapidly and efficiently delivered to the kidneys, its site of action and elimination. This targeted delivery makes it a "clean" physiological probe, as its clearance from the plasma is almost entirely a function of renal handling, without the confounding variable of widespread tissue distribution and metabolism that complicates the interpretation of clearance data for many other substances.

Property	Value	Source(s)
DrugBank ID	DB00345	1
Type	Small Molecule	3
CAS Number	61-78-9 (Acid); 94-16-6 (Sodium Salt)	1
IUPAC Name	(4-Aminobenzamido)acetic acid	1
Synonyms	para-Aminohippuric acid, PAH, PAHA	1
Molecular Formula	C9​H10​N2​O3​	1
Average Molecular Weight	194.19 g/mol	1
Appearance	White to light yellow crystalline powder	16
Melting Point	197-202 °C (with decomposition)	16
pKa	3.61 - 3.83	4
Solubility (Sodium Salt)	Water soluble (50 mg/mL)	12
SMILES	C1=CC(=CC=C1C(=O)NCC(=O)O)N	14
InChIKey	HSMNQINEKMPTIC-UHFFFAOYSA-N	1

Preclinical Pharmacology

The unique pharmacological profile of aminohippuric acid is centered entirely on its interaction with the kidney. It acts not by eliciting a biological response, but by serving as an inert tracer whose passage through the renal system reveals fundamental physiological processes.

Mechanism of Action

The clearance of aminohippuric acid from the plasma is the result of two sequential renal processes: glomerular filtration and active tubular secretion.[2] Like other small molecules not bound to plasma proteins, PAH is freely filtered across the glomerular capillaries into Bowman's space. However, filtration alone accounts for only a fraction of its total elimination. The vast majority of PAH that escapes filtration and continues into the peritubular capillaries is then actively transported from the blood into the tubular fluid by the epithelial cells of the proximal tubules.[3] This active secretory process is so efficient that PAH is almost completely removed from the blood that perfuses the secretory portions of the kidney within a single pass.[1]

Pharmacodynamics

The key pharmacodynamic characteristic of aminohippuric acid is its exceptionally high renal extraction ratio. In a healthy individual, this ratio is approximately 0.92, meaning that 92% of the PAH delivered to the kidneys via the renal artery is removed and excreted into the urine.[1] This near-complete extraction is the physiological principle that underpins its diagnostic applications.

Basis for Effective Renal Plasma Flow (ERPF) Measurement

Because the kidneys extract PAH from the plasma with such high efficiency, its total clearance rate from the systemic circulation serves as an excellent and reliable approximation of the total rate of plasma flow to the kidneys. This measured value is termed the Effective Renal Plasma Flow (ERPF), where "effective" acknowledges that a small fraction of renal blood flow perfuses non-secretory tissues (like the renal capsule and fat) and is therefore not "seen" by the secretory mechanism.[2] This measurement is typically performed under conditions of a continuous low-dose infusion, designed to maintain plasma PAH concentrations at a low level (1.0 to 2.0 mg/100 mL). At these concentrations, the secretory transporters are far from saturated, and the rate of clearance is primarily dependent on the rate of delivery to the kidney, i.e., the plasma flow rate.[3]

Basis for Tubular Transport Maximum (TmPAH) Measurement

In addition to measuring flow, PAH can be used to assess the functional capacity of the renal tubules. This is accomplished by increasing the intravenous infusion rate to achieve high plasma concentrations (40-60 mg/100 mL). At these levels, the amount of PAH delivered to the peritubular capillaries exceeds the capacity of the active transport proteins to secrete it. The secretory mechanism becomes saturated, and it operates at its maximal velocity. This maximal rate of transport is known as the tubular transport maximum for PAH (TmPAH​).[3] Since the total amount of PAH excreted is the sum of the amount filtered and the amount secreted,

TmPAH​ can be calculated by subtracting the filtered load from the total urinary excretion rate. This measurement provides a quantitative index of the functional tubular cell mass and is a valuable tool in physiological research.[3]

Interaction with Renal Transporters

The highly efficient active secretion of aminohippuric acid is not a random process but is mediated by specific protein transporters located on the basolateral (blood-facing) and apical (lumen-facing) membranes of the proximal tubule epithelial cells.[1] PAH is a classic substrate for the Organic Anion Transporter (OAT) family of proteins, which are responsible for handling a wide variety of endogenous and exogenous organic anions.

Specifically, the uptake of PAH from the peritubular blood into the tubular cells is primarily mediated by OAT1 (gene symbol SLC22A6) and OAT3 (gene symbol SLC22A8) located on the basolateral membrane.[5] The affinity of PAH for OAT1 is notably high, with a reported Michaelis-Menten constant (

Km​) of 14.3 µM in an oocyte expression system.[5] This high affinity ensures that the transporter can efficiently bind and internalize PAH even at the low plasma concentrations used for ERPF measurements. The exit of PAH from the tubular cell into the tubular lumen is less well-defined but is known to involve other transporters, including the human inorganic phosphate transporter (NPT1) and the apical multidrug resistance-associated protein 2 (MRP2), for which PAH is also a substrate.[5]

The molecular interaction of PAH with these high-capacity, high-affinity transporters is the direct and fundamental cause of its high renal extraction ratio. This relationship forms a crucial bridge between molecular-level pharmacology and organ-level physiology. The efficient binding and transport by OAT1 and other proteins is the mechanism that produces the physiological property of near-complete single-pass extraction. It is this property that, in turn, validates the use of PAH clearance as the gold standard for measuring ERPF. The reliability of the diagnostic test is therefore directly traceable to the molecular kinetics of these specific transporter proteins.

Furthermore, the knowledge that PAH is a substrate for a broad range of multispecific organic anion transporters—including OAT1, OAT3, NPT1, and MRP2—provides a powerful predictive framework for understanding its clinical pharmacology.[5] These transporters are promiscuous, handling numerous other drugs, metabolites, and toxins. Consequently, the principle of competitive inhibition dictates that PAH will be susceptible to a wide array of drug-drug interactions with other anionic compounds that share these secretory pathways. This mechanistic understanding elevates the discussion of drug interactions from a simple list of empirical observations to a predictable consequence of its fundamental transport biology.

Pharmacokinetics and Metabolism

The disposition of aminohippuric acid in the body is characterized by rapid distribution and extremely efficient elimination, a profile that is entirely dependent on renal function.

Absorption and Distribution

As aminohippuric acid is administered exclusively by the intravenous route for its diagnostic indications, parameters related to oral absorption are not applicable.[1] Following intravenous injection, the drug distributes within the body. The steady-state volume of distribution (

Vss​) has been measured to be between 16 and 18 L in both healthy volunteers and patients with chronic renal impairment.[22] This volume is consistent with distribution primarily within the extracellular fluid compartment, which aligns with its physicochemical properties of high water solubility and lipid insolubility, limiting its ability to penetrate into intracellular spaces.

Metabolism and Excretion

Aminohippuric acid undergoes a single primary metabolic transformation to N-acetyl-p-aminohippuric acid (acetyl-PAH).[22] The vast majority of the administered dose, however, is eliminated unchanged. Elimination occurs almost exclusively via the kidneys through the previously described combination of glomerular filtration and active tubular secretion.[5]

In healthy volunteers, the elimination of PAH is extremely rapid, with a mean elimination half-life (t1/2​) of less than 30 minutes. Consequently, about 50% of an injected dose is excreted in the urine within 30 minutes, with quantitative recovery achieved within 3 hours.[22] Over an 8-hour period, approximately 17% of the dose is recovered in the urine as the acetyl-PAH metabolite.[22]

In patients with chronic renal impairment, this pharmacokinetic profile is drastically altered. The elimination half-life of PAH is significantly prolonged to a mean of 72 minutes. The recovery of the drug in urine over 8 hours is incomplete (mean of 83.6% of the dose), and the proportion of the dose recovered as the acetyl-PAH metabolite increases substantially to 26.9%.[22] This shift towards greater metabolism is a direct consequence of the impaired renal clearance of the parent drug. When the primary route of elimination (renal excretion) is compromised, the drug remains in the systemic circulation for a longer period. This increased residence time provides a greater opportunity for secondary elimination pathways, such as metabolism, to act upon the available drug pool. This phenomenon illustrates a fundamental pharmacokinetic principle: the balance between different elimination pathways can be significantly altered in the presence of organ dysfunction.

The renal clearance of PAH in healthy volunteers is very high, averaging approximately 599 ± 115 mL/min/1.73m2 during the first hour after administration. In stark contrast, in patients with renal impairment, the clearance is dramatically reduced, falling from an initial value of 194 ± 83 mL/min/1.73m2 in the first hour to as low as 61 ± 19 mL/min/1.73m2 between 4 and 6 hours post-injection.[22] This profound difference in clearance between healthy and diseased states is the quantitative basis of its diagnostic power. The acetyl-PAH metabolite itself has distinct pharmacokinetic properties, including a longer half-life (49 minutes in healthy subjects, 153 minutes in renal patients) and a much larger apparent volume of distribution (mean of 65.5 L in healthy volunteers), suggesting it distributes more widely in the body than the parent compound.[22]

Pharmacokinetic Parameter	Healthy Volunteers	Patients with Renal Impairment	Source(s)
PAH Elimination Half-life (t1/2​)	< 30 min	72 min	22
Acetyl-PAH Elimination Half-life (t1/2​)	49 min	153 min	22
PAH Volume of Distribution (Vss​)	16 - 18 L	16 - 18 L	22
PAH Renal Clearance (1st hour)	599 ± 115 mL/min/1.73m2	194 ± 83 mL/min/1.73m2	22
% Dose Excreted as Acetyl-PAH (8 hr)	17.0%	26.9%	22

Clinical Applications and Therapeutic Context

The clinical use of aminohippuric acid is highly specialized, focusing almost exclusively on its role as a diagnostic agent for the quantitative assessment of renal function.

Primary Diagnostic Indications

The two principal indications for aminohippuric acid are the estimation of effective renal plasma flow and the measurement of the functional capacity of the renal tubular secretory mechanism.[3]

Estimation of Effective Renal Plasma Flow (ERPF)

This is the most common clinical application of aminohippuric acid. The procedure involves the intravenous administration of PAH via a continuous infusion designed to achieve and maintain a low, stable plasma concentration, typically targeted at 2 mg/dL (or 2 mg/100 mL).[4] Under these steady-state conditions, timed urine and blood samples are collected. The clearance of PAH is then calculated using the standard clearance formula:

ERPF=PPAH​UPAH​×V​

where UPAH​ is the concentration of PAH in the urine, V is the urine flow rate (volume per unit time), and PPAH​ is the concentration of PAH in the plasma.4 The resulting value provides a quantitative measure of the volume of plasma that flows through the kidneys per unit time and is a critical parameter in the evaluation of renal health and disease.2

Measurement of Tubular Secretory Capacity (TmPAH​)

While less common in routine clinical practice and more often employed in a research setting, aminohippuric acid can be used to measure the maximal transport capacity of the proximal tubules.[3] This procedure requires a high-rate intravenous infusion to elevate plasma PAH levels sufficiently (typically 40-60 mg/100 mL) to saturate the tubular secretory transporters.[4] The

TmPAH​ is then calculated as the difference between the total amount of PAH excreted in the urine per minute and the amount that was filtered at the glomerulus per minute. This requires a simultaneous measurement of the glomerular filtration rate (GFR), for which inulin or another suitable marker is generally used.[3] The

TmPAH​ value reflects the functional mass of the proximal tubules and can be reduced in diseases that damage this part of the nephron.

Historical Pharmaceutical Utility

Beyond its diagnostic role, aminohippuric acid has a notable place in pharmaceutical history. During World War II, when supplies of the newly discovered antibiotic penicillin were scarce and precious, PAH was strategically co-administered with it.[1] The rationale for this combination was based on a shared mechanism of elimination. Both penicillin and PAH are organic anions that are actively secreted by the same transport systems in the renal tubules. By administering a high dose of PAH, clinicians could competitively inhibit the renal secretion of penicillin. This competition effectively reduced the rate of penicillin clearance from the body, thereby prolonging its elimination half-life and maintaining therapeutic blood concentrations for longer periods. This application allowed for more effective antibacterial therapy with lower or less frequent doses of the limited antibiotic supply.[1] This historical use serves as the archetypal clinical example of competitive inhibition at a renal transporter, perfectly illustrating the pharmacological principle that underpins the majority of PAH's modern, clinically significant drug-drug interactions. It provides a powerful and tangible link between fundamental pharmacological theory and impactful clinical practice.

Dosage, Administration, and Monitoring

Aminohippuric acid must be administered intravenously; single injection techniques are generally considered inaccurate for clearance measurements, necessitating the use of a constant infusion to achieve a steady-state plasma concentration.[2]

For the measurement of ERPF, the standard protocol involves an initial priming or loading dose of 6-10 mg/kg, administered to rapidly achieve the target plasma level. This is immediately followed by a continuous maintenance infusion at a rate of 10-24 mg/min.[4] The precise rate is adjusted to maintain the plasma PAH concentration at the target of 2 mg/100 mL. During the procedure, particularly during the high-dose infusions required for

TmPAH​ measurement, the patient should be continuously and closely monitored for the development of any adverse reactions.[4]

Investigational Uses and Clinical Trials

The utility of aminohippuric acid as a precise physiological probe continues in modern medical research, where it is used to elucidate the renal effects of new therapeutic agents. For instance, a recruiting Phase 4 clinical trial (NCT04649229) is utilizing PAH clearance, in conjunction with iohexol clearance for GFR, to investigate the mechanistic underpinnings of the hypotensive response to angiotensin receptor-neprilysin inhibitors (ARNIs) in patients with cardiac failure.[25] Additionally, a now-withdrawn clinical trial (NCT01702688) had planned to use PAH to assess renal function during the evaluation of a novel lysine-specific demethylase 1 inhibitor in the context of hypertension.[26] These examples demonstrate the enduring value of this legacy diagnostic tool. Even as highly sophisticated new drugs are developed, PAH remains an essential instrument for understanding their impact on renal hemodynamics, a critical aspect of the complex interplay between the cardiovascular and renal systems.

Safety, Tolerability, and Risk Management

While the aminohippuric acid molecule itself is considered essentially nontoxic at the concentrations used for diagnostic tests, its administration is associated with a distinct profile of potential adverse effects and requires careful consideration of patient-specific risk factors.[3]

Adverse Drug Reactions

The adverse effects associated with aminohippurate sodium can be categorized into common infusion-related phenomena and less common but more serious hypersensitivity reactions.

• Common/Infusion-Related Effects: Many patients experience transient vasomotor disturbances during or shortly after the infusion begins. These include a sensation of warmth, flushing (redness of the face, neck, and upper chest), and tingling of the skin.[4] A sudden desire to defecate or urinate is also a frequently reported sensation.[7] Gastrointestinal symptoms such as nausea, vomiting, and muscle cramps may also occur.[7] Other reported effects include dizziness and light-headedness.[7]
• Hypersensitivity Reactions: These represent the most significant safety concern. Allergic reactions can manifest across a spectrum of severity. Milder reactions may involve cutaneous symptoms such as skin rash, itching (pruritus), hives (urticaria), and welts.[2] More severe, systemic reactions can be life-threatening and include angioedema (rapid swelling of the face, lips, tongue, and throat, which can compromise the airway), respiratory distress (characterized by cough, wheezing, shortness of breath, and a sensation of tightness in the chest), and full-blown anaphylaxis with cardiovascular collapse.[4]

The overall safety profile of aminohippuric acid reveals a critical dichotomy. The drug substance is repeatedly described as "essentially nontoxic" at diagnostic concentrations, suggesting a lack of direct, dose-dependent organ toxicity.[3] However, its clinical administration carries notable procedural and immunological risks. The primary dangers stem not from the molecule's intrinsic chemical properties, but from the physiological stress induced by the procedure and the potential for an idiosyncratic immune response. The risk in patients with heart disease is directly linked to the rapid increase in plasma volume from the intravenous fluid load, while the risk of anaphylaxis is an unpredictable immunological event. This understanding shifts the focus of clinical risk management away from the drug's chemistry and toward careful patient selection, thorough allergy history assessment, and vigilant monitoring during the administration process.

Contraindications, Warnings, and Precautions

Safe use of aminohippurate sodium requires adherence to specific contraindications and warnings.

• Contraindications: The primary contraindication is a history of a hypersensitivity reaction to aminohippurate sodium or any of its components.[23]
• Warnings and Precautions:
• Low Cardiac Reserve and Congestive Heart Failure: Intravenous solutions must be administered with significant caution in patients with pre-existing heart disease or low cardiac reserve. The rapid increase in plasma volume associated with the infusion can overwhelm a compromised cardiovascular system and precipitate or exacerbate congestive heart failure.[4]
• Latex Allergy: A specific, FDA-mandated warning concerns the packaging of the drug. The vial stopper contains dry natural rubber, a derivative of latex, which can cause allergic reactions in individuals with latex sensitivity. Clinicians must be aware of a patient's latex allergy status before administration.[4]

Guidance for Use in Specific Populations

Data on the use of aminohippurate sodium in certain populations are limited, necessitating a cautious approach.

• Pregnancy: Aminohippurate sodium is assigned to Pregnancy Category C. There have been no animal reproduction studies conducted with the drug, and it is not known whether it can cause fetal harm when administered to a pregnant woman or affect reproductive capacity. Therefore, it should be given during pregnancy only if the potential benefit justifies the potential risk to the fetus.[4]
• Lactation: It is not known whether aminohippuric acid is excreted in human milk. Because many drugs are, caution should be exercised when the test is performed on a nursing woman.[4]
• Pediatric Use: The safety and effectiveness of aminohippurate sodium in pediatric patients have not been established.[7]
• Geriatric Use: To date, appropriate studies have not demonstrated any geriatric-specific problems that would limit the usefulness of aminohippurate sodium in the elderly. Clinical experience has not identified consistent differences in response between elderly and younger patients.[7]

Clinically Significant Drug-Drug Interactions

The interaction profile of aminohippuric acid is extensive and mechanistically consistent. The vast majority of clinically significant interactions are pharmacokinetic in nature, arising from competition for the active transport systems in the renal proximal tubules, primarily the organic anion transporters (OATs).[1]

Interactions Affecting PAH Clearance

Many drugs can interfere with the renal secretion of PAH, thereby decreasing its clearance and leading to erroneously low calculated values for ERPF and TmPAH​. This can confound the diagnostic interpretation of the test.

• Probenecid: This uricosuric agent is the classic and most potent inhibitor of OAT-mediated tubular secretion. Its co-administration will strongly depress PAH secretion, rendering the results of the clearance test invalid.[4]
• Antibiotics: Numerous antibiotics, particularly beta-lactams like penicillins and cephalosporins, are also OAT substrates and compete with PAH for secretion. Examples include benzylpenicillin, cefalotin, and cefamandole.[3]
• Nonsteroidal Anti-inflammatory Drugs (NSAIDs) and Salicylates: This class of drugs, including aspirin and aceclofenac, are organic anions that compete for tubular clearance and can increase plasma PAH levels by reducing its excretion.[23]
• Other Anionic Drugs: A wide range of other drugs can interfere via the same mechanism. These include diuretics (e.g., bendroflumethiazide), antivirals (e.g., acyclovir), and immunosuppressants/chemotherapeutics like methotrexate and mycophenolate.[23]

Interactions Where PAH Affects Other Drugs

By the same principle of competitive inhibition, aminohippuric acid can decrease the renal clearance of other OAT substrates, potentially increasing their plasma concentrations and risk of toxicity. The historical use of PAH to prolong the half-life of penicillin is the canonical example of this type of interaction.[1] Other drugs whose serum concentrations may be increased by co-administration with PAH include amantadine, dalfampridine, and dopamine.[3]

Analytical Interferences

A separate class of interaction is analytical, where a co-administered drug does not affect the physiology of PAH transport but interferes with the laboratory method used to measure its concentration. Compounds such as sulfonamides, the local anesthetic procaine (which is structurally related to PAH), and thiazolesulfone interfere with the chemical color development step of the common Bratton-Marshall assay for PAH. Their presence makes it impossible to obtain an accurate measurement of PAH concentration in plasma or urine.[4]

The comprehensive interaction profile of aminohippuric acid can be viewed not just as a list of clinical precautions, but as a functional map of the OAT system's activity in vivo. By observing which drugs interfere with the clearance of this probe substrate, one can infer that those drugs are also substrates or inhibitors of the OAT pathway. This extends the utility of PAH beyond a simple diagnostic for renal flow; it can also be employed as a powerful probe drug in clinical pharmacology studies. For instance, in the development of a new anionic drug candidate, a drug-drug interaction study with PAH can be conducted. A significant decrease in PAH clearance upon co-administration would provide strong evidence that the new chemical entity is also actively secreted by the kidneys via the OAT system, yielding crucial information about its likely route of elimination and potential for other renal drug interactions.

Interacting Drug / Class	Mechanism of Interaction	Clinical Consequence	Source(s)
Probenecid	Potent competitive inhibition of OAT-mediated tubular secretion.	Depresses PAH secretion, leading to falsely low and invalid ERPF and TmPAH​ values.	4
Penicillins & Cephalosporins	Competitive inhibition of OAT-mediated tubular secretion.	Decreased renal clearance of both PAH and the antibiotic. Can lead to falsely low ERPF values and increased antibiotic levels.	1
NSAIDs & Salicylates	Competitive inhibition of OAT-mediated tubular secretion.	Decreased PAH clearance, leading to falsely low ERPF values.	23
Methotrexate, Mycophenolate	Competitive inhibition of OAT-mediated tubular secretion.	Decreased PAH clearance. Potential for increased levels of the co-administered drug.	23
Sulfonamides, Procaine	Analytical interference with the colorimetric assay used to measure PAH concentration.	Inability to accurately measure PAH levels in plasma and urine, making the test results invalid.	4
Amantadine, Dalfampridine	PAH competitively inhibits the renal secretion of these drugs.	Increased serum concentration and potential for toxicity of the co-administered drug.	3

Non-Clinical Toxicology and Synthesis

Toxicological Summary

The non-clinical toxicological data for aminohippuric acid is limited, and information from various sources, such as Safety Data Sheets (SDS), presents some inconsistencies.

• Acute Toxicity: Several SDS classify the substance as non-hazardous according to the Globally Harmonized System (GHS).[19] However, other sources classify it as harmful if swallowed (Acute toxicity, oral, Category 4), a skin irritant (Category 2), a serious eye irritant (Category 2A), and a respiratory tract irritant (Category 3).[32] These discrepancies may be attributable to the physical form of the chemical being assessed (e.g., a fine, easily aerosolized powder versus a solution) or the application of different classification criteria. A specific median lethal dose ( LD50​) for aminohippuric acid is not provided in the available materials. For its parent compound, hippuric acid, an intraperitoneal LD50​ of 1,500 mg/kg in rats has been reported.[32]
• Carcinogenesis, Mutagenesis, and Impairment of Fertility: There is a notable absence of long-term animal studies designed to evaluate the carcinogenic potential, mutagenic effects, or impact on fertility of aminohippuric acid.[4] Based on the available data, it is not classified as a carcinogen, mutagen, or reproductive toxicant.[19]

The absence of comprehensive long-term toxicology data is a significant information gap when viewed through the lens of modern drug development standards. However, this is not uncommon for legacy drugs that were approved well before current regulatory requirements were established. Aminohippuric acid's long history of clinical use, primarily as a single-administration diagnostic agent rather than a chronically administered therapeutic drug, has likely provided a de facto body of evidence for its safety in its intended acute-use context. The risk-benefit assessment for a diagnostic agent used infrequently is fundamentally different from that for a therapeutic drug taken daily. Consequently, the lack of chronic toxicity studies is likely a reflection of its historical context and specific, limited clinical application, for which the risk of unknown long-term effects has been considered acceptably low.

Chemical Synthesis

Aminohippuric acid is structurally a derivative of hippuric acid.[1] Its synthesis, both biochemically and industrially, involves the formation of an amide bond between p-aminobenzoic acid (PABA) and the amino acid glycine.[1] In vivo, this conjugation reaction is known to occur in the liver.[12] Industrial synthesis strategies can employ similar principles, for example, by reacting a mixed anhydride of an N-acylamino acid with an aminobenzoic acid to form the desired product.[34]

Regulatory and Development History

Aminohippuric acid has a long and established history as a diagnostic agent in medicine, with regulatory approval spanning many decades.

Approval History and Status

Aminohippuric acid is an approved small molecule drug.[3] Its initial approval by the U.S. Food and Drug Administration (FDA) dates back to 1944, establishing it as a legacy drug that has been a part of the medical armamentarium for nearly a century.[9] Its sodium salt, aminohippurate sodium, is listed as an active ingredient in FDA-approved drug products.[36] It is classified under the Anatomical Therapeutic Chemical (ATC) classification system with the code V04CH30, which designates agents used for tests of renal function.[1]

Regulatory Oversight and Labeling Updates

Despite its age, the drug remains under active regulatory oversight to ensure its safety information reflects current medical knowledge. A clear example of this is the Supplemental New Drug Application (sNDA) submitted by Merck Sharp & Dohme Corp., which was approved by the FDA on March 18, 2011.[30] The purpose of this sNDA was to update the "Precautions" section of the product labeling to include a specific warning about the presence of dry natural latex rubber in the vial stopper and the associated risk of allergic reactions in latex-sensitive individuals.[30] This action highlights the dynamic nature of drug safety and regulation. It demonstrates that even for very old, well-established products, post-marketing surveillance and evolving standards of care can, and do, lead to important labeling changes designed to protect specific patient populations. This process ensures that the regulatory framework for even the oldest drugs adapts to new safety information as it becomes available.

Conclusion and Future Directions

Aminohippuric acid stands as a remarkable example of a legacy diagnostic agent whose clinical and scientific relevance has endured for decades. Its utility is derived from a unique and highly efficient renal clearance mechanism, which combines glomerular filtration with near-complete active tubular secretion. This profile allows its plasma clearance to serve as the undisputed in-vivo gold standard for the measurement of effective renal plasma flow and as a tool for quantifying the maximal functional capacity of the renal tubules.

This comprehensive analysis has illuminated the intricate connections between the drug's fundamental properties and its clinical applications. Its physicochemical characteristics—high water solubility as a sodium salt and lipid insolubility—are not merely descriptive facts but are the key enablers of its formulation as an intravenous agent that remains confined to the plasma for efficient delivery to the kidneys. At a molecular level, its high-affinity interaction with a suite of organic anion transporters, particularly OAT1, is the direct cause of its high renal extraction. This molecular behavior, in turn, defines not only its diagnostic utility but also its predictable profile of drug-drug interactions, which are almost entirely based on competition for this shared secretory pathway. Furthermore, its safety profile is dictated less by intrinsic chemical toxicity and more by the procedural risks of intravenous infusion and the potential for immunological hypersensitivity, shifting the focus of risk management to patient selection and careful administration.

From its strategic use in optimizing antibiotic therapy during World War II to its contemporary role as a critical physiological probe in cutting-edge cardiovascular research, aminohippuric acid has consistently proven its value. It remains an indispensable tool in clinical nephrology for the precise evaluation of renal function and in renal physiology for the continued elucidation of the complex roles of renal transporters in health and disease. Future directions will undoubtedly see its continued application in clinical trials to characterize the renal hemodynamic effects of novel therapeutics, ensuring that this venerable diagnostic agent will continue to contribute to medical advancement for the foreseeable future.

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