An In-Depth Analysis of Carotuximab (DB06322): Mechanism, Clinical Development, and Therapeutic Potential

Executive Summary

Carotuximab is an investigational chimeric monoclonal antibody that represents a novel therapeutic approach in oncology by targeting endoglin (CD105), a co-receptor in the transforming growth factor-beta (TGF-β) signaling superfamily critical for angiogenesis. This report provides a comprehensive analysis of Carotuximab's molecular profile, its intricate mechanism of action, its extensive and varied clinical development history, and its future therapeutic potential. Initially developed under the code name TRC-105, Carotuximab's journey is a compelling narrative of scientific promise, significant clinical setbacks, and recent, potentially transformative, success.

The drug's mechanism is multifaceted, extending beyond simple anti-angiogenesis. By binding to endoglin, which is highly expressed on proliferating tumor endothelial cells, Carotuximab inhibits the pro-angiogenic Smad1/5/8 signaling pathway. Furthermore, it demonstrates crucial crosstalk with the vascular endothelial growth factor (VEGF) pathway, potentiating the effects of VEGF inhibitors and providing a strong rationale for combination therapies. Its IgG1 backbone also enables immunomodulatory functions, including antibody-dependent cell-mediated cytotoxicity (ADCC).

This robust preclinical rationale led to a broad clinical development program across numerous solid tumors. However, the program met a pivotal failure with the termination of the Phase 3 TAPPAS trial in advanced angiosarcoma, where Carotuximab failed to improve progression-free survival when added to the potent VEGF inhibitor pazopanib. This result led to a strategic re-evaluation of the asset. The narrative of Carotuximab has recently been revitalized by highly promising interim data from an ongoing Phase 2 trial in metastatic castration-resistant prostate cancer (mCRPC). In this setting, Carotuximab is being evaluated not as a general anti-angiogenic agent, but as a targeted therapy to reverse a specific mechanism of acquired resistance to androgen receptor inhibitors, which involves the upregulation of CD105.

Carotuximab has a consistent and manageable safety profile, with the most common adverse events—anemia, telangiectasias, and minor bleeding—being predictable on-target effects related to its mechanism. After a convoluted corporate history, the asset is now under the stewardship of Kairos Pharma. The future of Carotuximab hinges on the final results of the mCRPC trial, which could validate its role as a resistance-reversal agent and establish a new therapeutic paradigm in prostate cancer, fundamentally reshaping the drug's value proposition and clinical trajectory.

Molecular Profile and Physicochemical Properties

Carotuximab is a biopharmaceutical drug, classified as a human/murine chimeric immunoglobulin G1 (IgG1) kappa monoclonal antibody.[1] As a chimeric antibody, its structure is a composite of genetic material from two different species. The variable regions of the heavy and light chains, which are responsible for antigen recognition and binding, are derived from a murine (mouse) source. The constant regions of the antibody are derived from human immunoglobulin sequences.[3] This chimerization process is a standard technique in antibody engineering designed to reduce the immunogenicity of a purely murine antibody when administered to human patients, thereby decreasing the risk of a human anti-mouse antibody (HAMA) response and improving the antibody's pharmacokinetic profile and effector functions.

The production of chimeric antibodies like Carotuximab involves advanced recombinant DNA technology. The process generally begins with the isolation of mRNA from the murine hybridoma cell line that produces the desired anti-endoglin antibody. The genes encoding the variable heavy (VH​) and variable light (VL​) regions are then amplified via polymerase chain reaction (PCR) and cloned into expression vectors that already contain the DNA sequences for the human IgG1 heavy chain and kappa light chain constant regions.[5] These final expression vectors are then transfected into a suitable mammalian host cell line, typically Chinese Hamster Ovary (CHO) cells, which are capable of the complex post-translational modifications, such as glycosylation, required for a functional antibody.[6] The transfected cells are cultured in large-scale bioreactors, from which the secreted Carotuximab antibody is harvested and subjected to a multi-step purification process to yield the final, highly pure therapeutic product.[8]

While a specific three-dimensional crystal structure for Carotuximab is not publicly available in the Protein Data Bank (PDB), the general architecture is well-understood and follows the canonical 'Y'-shape of an IgG1 antibody.[9] The structures of other therapeutic chimeric antibodies, such as rituximab (anti-CD20), provide a relevant architectural model for its overall form and function.[10] The fundamental properties and identifiers for Carotuximab are consolidated in Table 1.

Table 1: Key Identifiers and Properties of Carotuximab

Property	Detail	Source(s)
Drug Name	Carotuximab	4
DrugBank ID	DB06322	12
Type	Biotech, Chimeric (mouse/human) Monoclonal Antibody (IgG1 kappa)	2
Synonyms/Code Names	TRC-105, TRC105, DE-122, c-SN6j, ENV105	2
CAS Number	1268714-50-6	4
UNII	YB2EWE6139	4
Molecular Formula	C6420​H9922​N1718​O2010​S46​	4
Molar Mass	144808.77 g·mol−1; ~145 kDa	4
Target	Endoglin (CD105)	3

Mechanism of Action: Targeting the Endoglin (CD105) Pathway

The therapeutic rationale for Carotuximab is centered on its ability to specifically inhibit endoglin (gene: ENG), a key regulator of vascular biology and tumor progression. Its mechanism of action is complex, involving direct inhibition of pro-angiogenic signaling, modulation of the tumor microenvironment's response to other therapies, and engagement of the immune system.

The Role of Endoglin (CD105) in Pathophysiology

Endoglin, also known as CD105, is a 180 kDa homodimeric transmembrane glycoprotein that functions as a crucial accessory or co-receptor for the transforming growth factor-beta (TGF-β) superfamily of ligands.[3] While its expression is relatively low on the surface of quiescent, mature endothelial cells, it is significantly upregulated on proliferating endothelial cells.[1] This selective overexpression occurs in physiological settings of active blood vessel formation (angiogenesis), such as embryogenesis and wound healing, and pathologically in the tumor neovasculature.[1] The dense expression of CD105 on tumor blood vessels has been identified as a negative prognostic marker in a wide range of solid tumors, including breast, lung, prostate, and renal cancers, correlating with increased metastasis and decreased patient survival.[1] This makes endoglin an ideal and highly specific target for anti-cancer therapies designed to disrupt tumor angiogenesis.

Carotuximab's Molecular Engagement and Downstream Signaling

Carotuximab is a high-affinity monoclonal antibody that binds directly to the extracellular domain of human endoglin, with a dissociation constant (KD​) of 1-2 ng/mL.[1] This binding event is the primary trigger for its therapeutic effects. Endoglin itself lacks intrinsic kinase activity; instead, it modulates signaling by forming heterotypic receptor complexes with other TGF-

β family receptors, namely the type I receptors (activin-receptor like kinases, or ALKs) and type II receptors.[17]

In endothelial cells, a key signaling axis involves the ligand bone morphogenetic protein 9 (BMP9), which binds to the receptor complex containing endoglin and the type I receptor ALK-1.[1] This ligand binding event leads to the phosphorylation and activation of ALK-1, which in turn phosphorylates the downstream effector proteins Smad1, Smad5, and Smad8 (Smad1/5/8).[17] The phosphorylated Smad1/5/8 complex then associates with Smad4, translocates to the nucleus, and activates the transcription of genes essential for endothelial cell proliferation, migration, and survival—the cellular hallmarks of angiogenesis.[17]

Carotuximab's primary mechanism is the direct inhibition of this pathway. By binding to endoglin, it prevents the binding of ligands like BMP9 and disrupts the formation of the functional receptor complex, thereby blocking the downstream phosphorylation of Smad1/5/8.[1]

In vitro experiments have confirmed that treatment with Carotuximab effectively reduces the levels of phosphorylated Smad1/5 and Smad2/3 in endothelial cells, providing direct evidence of its ability to shut down this pro-angiogenic signaling cascade.[16]

Crosstalk with the VEGF Pathway: A Rationale for Synergy and Resistance Reversal

A critical aspect of Carotuximab's mechanism is its interplay with the vascular endothelial growth factor (VEGF) pathway, the most well-characterized driver of tumor angiogenesis and the target of established therapies like bevacizumab. The endoglin and VEGF pathways are not independent but are deeply interconnected. Endoglin has been shown to form a physical complex with VEGF receptor 2 (VEGFR2) on the endothelial cell surface, an interaction that stabilizes VEGFR2 and prevents its lysosomal degradation.[18] This effectively enhances and prolongs VEGF-mediated signaling.

This biological link provides a compelling rationale for a dual-targeting strategy. Carotuximab has been shown to directly inhibit endothelial cell proliferation induced by both VEGF and basic fibroblast growth factor (bFGF) and to promote the degradation of VEGFR2.[18] The combination of an endoglin inhibitor with a VEGF inhibitor is therefore hypothesized to produce a more profound and durable anti-angiogenic effect than either agent alone.

This synergy becomes particularly relevant in the context of acquired resistance to anti-VEGF therapy. Clinical observations and preclinical models have shown that endoglin expression is often upregulated on tumor endothelial cells as a compensatory escape mechanism following treatment with VEGF inhibitors.[18] As tumors become less dependent on the VEGF pathway, they may become more reliant on the endoglin/ALK-1 pathway for survival and angiogenesis. This biological shift creates a therapeutic vulnerability. It reframes Carotuximab's role from simply being an alternative anti-angiogenic agent to being a targeted therapy designed to counteract a specific, acquired resistance mechanism. This hypothesis has been a driving force behind much of Carotuximab's clinical development, particularly its investigation in patients with tumors refractory to prior anti-VEGF therapy, such as in glioblastoma and renal cell carcinoma.[18]

Immunomodulatory Functions: Beyond Anti-Angiogenesis

As an IgG1 isotype antibody, Carotuximab's mechanism extends beyond simple receptor blockade to include the active engagement of the immune system.[1] The Fc portion of the IgG1 antibody can be recognized by Fc gamma receptors (Fc$\gamma$R) on the surface of immune effector cells, such as natural killer (NK) cells, neutrophils, and macrophages. This engagement triggers antibody-dependent cell-mediated cytotoxicity (ADCC), a process in which the immune cells release cytotoxic granules that lyse the antibody-coated target cell.[1] In the context of Carotuximab, this means it can direct the immune system to identify and destroy the CD105-expressing endothelial cells that form the tumor's blood supply.

Furthermore, preclinical studies have revealed a more nuanced immunomodulatory role. Endoglin is also expressed on certain immune cell populations, including a subset of immunosuppressive regulatory T cells (Tregs) within the tumor microenvironment.[17] Carotuximab was shown to be more effective in suppressing tumor growth in immune-competent mice compared to immunodeficient mice, and this effect was dependent on CD8+ T-cells. The antibody appeared to selectively deplete the intratumoral Treg population, thereby removing a key brake on the anti-tumor immune response.[17]

This suggests a dual-pronged attack on the tumor microenvironment. On one hand, Carotuximab disrupts the tumor vasculature, cutting off the supply of oxygen and nutrients. On the other hand, it remodels the immune landscape within the tumor, removing immunosuppressive cells and enhancing the ability of cytotoxic T-cells to attack cancer cells directly. This dual mechanism implies that Carotuximab's full potential may be realized in combination not only with other anti-angiogenic agents but also with immunotherapies such as checkpoint inhibitors, representing a compelling area for future investigation.

Preclinical and Clinical Pharmacology

The pharmacological profile of Carotuximab has been characterized through a series of in vitro experiments, in vivo animal models, and human clinical trials, which collectively define its biological activity and its behavior within the body.

In Vitro and In Vivo Pharmacodynamic Studies

In vitro, Carotuximab has consistently demonstrated potent anti-angiogenic activity. It effectively inhibits the proliferation of human umbilical vein endothelial cells (HUVECs) stimulated by key pro-angiogenic factors, including VEGF and bFGF.[18] Beyond inhibiting proliferation, Carotuximab also interferes with endothelial cell function. Studies using human aortic endothelial cells (HAoECs) have shown that Carotuximab can prevent the adhesion and subsequent transmigration of monocytes through the endothelial monolayer.[16] This effect, observed in models of hypercholesterolemia and hyperglycemia, suggests a broader potential for Carotuximab in modulating endothelial dysfunction beyond the context of cancer.

In vivo animal studies have corroborated these findings. In xenograft models of human tumors, Carotuximab has been shown to inhibit tumor growth and angiogenesis.[17] In a model of acute myeloid leukemia (AML), the combination of Carotuximab (2 mg/kg intravenously every 3 days) with the chemotherapeutic agent Decitabine resulted in a more durable anti-leukemic effect than either agent alone, highlighting its potential in hematologic malignancies.[19]

Pharmacokinetic Profile in Clinical Studies

Pharmacokinetic (PK) data from human trials have been crucial in establishing a safe and effective dosing regimen for Carotuximab. The first-in-human Phase I study in patients with advanced solid tumors provided key insights into its absorption, distribution, metabolism, and excretion (ADME) profile.[24] As a large protein administered intravenously, its absorption is immediate and complete. The study demonstrated that Carotuximab exposure, measured by serum concentration, increased in a dose-proportional manner.[24]

The elimination of Carotuximab, like many therapeutic monoclonal antibodies, is complex. Pharmacokinetic modeling suggests a two-compartment model with both linear (first-order) and nonlinear, saturable (zero-order) elimination pathways.[26] The linear pathway represents nonspecific clearance through the reticuloendothelial system, while the nonlinear pathway reflects target-mediated drug disposition (TMDD), where the antibody is cleared by binding to its target, CD105, on cell surfaces and being internalized.

A key objective of the Phase I study was to identify a dosing schedule that could maintain serum concentrations of Carotuximab at a level sufficient to continuously saturate the CD105 receptors on target endothelial cells. The study found that this pharmacodynamic goal was achieved with dosing regimens of 10 mg/kg administered weekly or 15 mg/kg administered every two weeks.[24] This PK/PD relationship directly informed the rational design of dosing schedules in subsequent Phase II and III trials. For example, the pivotal TAPPAS trial utilized a 10 mg/kg weekly dose [1], while the ongoing mCRPC trial employs a loading dose strategy followed by 15 mg/kg every four weeks, a regimen consistent with the long half-life expected of an IgG1 antibody and the exposure levels found to be effective for receptor saturation.[24] Importantly, the current formulation of Carotuximab, produced in CHO cells, has not been associated with the development of anti-drug antibodies, indicating low immunogenicity.[24]

Clinical Development Program: A Comprehensive Review

Carotuximab has undergone an extensive and varied clinical development program, investigating its utility across a wide spectrum of oncologic and ophthalmologic indications. This history is marked by both significant failures that prompted strategic reassessments and recent promising results that have opened new avenues for development. A summary of the major clinical trials is presented in Table 2.

Table 2: Summary of Major Clinical Trials for Carotuximab

Trial ID (NCT)	Phase	Indication	Combination Agent(s)	Primary Endpoint	Key Outcome / Status	Source(s)
NCT02979899 (TAPPAS)	3	Advanced Angiosarcoma	Pazopanib	Progression-Free Survival (PFS)	Did not improve PFS; Terminated for futility	1
NCT05534646	2	Metastatic Castration-Resistant Prostate Cancer (mCRPC)	Apalutamide	Radiographic PFS (rPFS)	Ongoing; Positive interim data (median PFS >13 months)	27
NCT01564914 (ENDOT)	2	Bevacizumab-Refractory Glioblastoma (GBM)	Bevacizumab (optional)	Overall Survival (OS)	Modest activity with combination (median OS 5.7 months)	23
NCT01806064 (TRAXAR)	2	Advanced Renal Cell Carcinoma (RCC)	Axitinib	Not specified	Completed; Showed tolerability and some activity	1
NCT02555306 (PAVE)	1	Exudative Age-Related Macular Degeneration (AMD)	Monotherapy (intravitreal)	Safety & Tolerability	Generally well-tolerated; Development discontinued	18
NCT03780010	1	Non-Small Cell Lung Cancer (NSCLC)	Paclitaxel/Carboplatin/Bevacizumab	Not specified	Completed	32
NCT01975519	1/2	Advanced Soft Tissue Sarcoma	Pazopanib	Not specified	Completed	33
NCT02560779	1/2	Hepatocellular Carcinoma	Sorafenib	Not specified	Completed	19

Angiosarcoma: The TAPPAS Trial (NCT02979899) and a Pivotal Failure

The Phase 3 TAPPAS trial was the most advanced study in Carotuximab's development program and represented a significant strategic investment. The choice of angiosarcoma, a rare and aggressive sarcoma of endothelial cell origin, was based on a strong biological rationale: a tumor composed of the very cells that highly express the drug's target, CD105.[1] The trial was an open-label, randomized study comparing the combination of Carotuximab (10 mg/kg weekly) plus the standard-of-care VEGF receptor tyrosine kinase inhibitor (TKI) pazopanib versus pazopanib alone.[1]

Despite the compelling premise, the trial was terminated for futility in April 2019 after an interim analysis determined it was unlikely to meet its primary endpoint.[28] The final published results confirmed this outcome, showing no statistically significant improvement in progression-free survival (PFS). The median PFS was 4.2 months for the combination arm compared to 4.3 months for the pazopanib monotherapy arm (Hazard Ratio 0.98).[29] This definitive negative result was a major setback for the program. The failure suggests that in a tumor type already highly dependent on and sensitive to potent VEGF pathway blockade (as provided by pazopanib), the addition of an endoglin inhibitor may offer redundant, rather than synergistic, anti-angiogenic effects. The anti-angiogenic "ceiling" may have already been reached by pazopanib, leaving no therapeutic window for Carotuximab to provide additional benefit. This outcome forced a critical re-evaluation of Carotuximab's development strategy, shifting focus away from indications of primary angiogenesis toward those defined by specific resistance mechanisms.

Metastatic Castration-Resistant Prostate Cancer (mCRPC): A Potential Second Act (NCT05534646)

In a remarkable strategic pivot, Carotuximab is now being investigated in a Phase 2 trial for mCRPC, where its mechanism is being leveraged to address a different biological problem: acquired drug resistance.[27] The scientific basis for this trial is the observation that prostate cancer cells that become resistant to potent androgen receptor (AR) signaling inhibitors, like apalutamide, often upregulate CD105 as a pro-survival escape mechanism.[35] The hypothesis is that by blocking this CD105-mediated signaling with Carotuximab, it may be possible to re-sensitize the tumors to AR inhibition.

The trial (NCT05534646) is evaluating Carotuximab in combination with apalutamide in patients with mCRPC who have progressed on prior AR-targeted therapies.[27] Interim results from the study have been exceptionally promising. In an initial cohort of eight evaluable patients, the median radiographic PFS was reported to be over 13 months, a substantial improvement over the expected PFS of 3.7-5.6 months with standard hormone therapies in this setting.[27] Furthermore, the combination was well-tolerated, with no new safety signals.[27]

This trial represents a fundamental shift in the positioning of Carotuximab, moving it from the category of a general anti-angiogenic agent to a precision therapy aimed at reversing a specific mechanism of drug resistance. The strong preliminary efficacy, in stark contrast to the TAPPAS failure, underscores the importance of applying targeted agents to solve well-defined biological problems. If these results are confirmed in the full study and subsequent pivotal trials, it could represent a major breakthrough for patients with mCRPC and completely revitalize the Carotuximab program.

Glioblastoma (GBM) and Other CNS Cancers (NCT01564914)

The Phase 2 ENDOT trial investigated Carotuximab in patients with recurrent glioblastoma, a notoriously difficult-to-treat brain tumor with a dismal prognosis, particularly after failure of standard therapy including the VEGF inhibitor bevacizumab.[23] This trial directly tested the hypothesis that Carotuximab could have activity in tumors that had developed resistance to anti-VEGF therapy. The study evaluated Carotuximab monotherapy and Carotuximab in combination with bevacizumab in bevacizumab-refractory patients.[23] The combination therapy resulted in a median overall survival of 5.7 months.[23] While modest, this outcome is noteworthy in a patient population where survival is typically measured in weeks, providing clinical proof-of-concept for Carotuximab's activity in the VEGF-refractory setting.

Renal Cell Carcinoma (RCC) and Other Solid Tumors

Given the importance of angiogenesis in renal cell carcinoma, Carotuximab was investigated in this indication, receiving a Fast Track designation from the FDA in 2015.[1] The Phase 2 TRAXAR study (NCT01806064) evaluated Carotuximab in combination with the VEGF TKI axitinib.[1] An earlier Phase 1b dose-escalation portion of the study found that the combination was well-tolerated and produced decreases in tumor burden in several heavily pre-treated, VEGFR TKI-refractory patients.[31] Carotuximab has also been studied in early-phase trials for other solid tumors, including hepatocellular carcinoma (in combination with sorafenib) and non-small cell lung cancer (in combination with chemotherapy and bevacizumab), as part of a broad initial exploration of its anti-cancer activity.[30]

Ophthalmology (Exudative AMD)

An ophthalmic formulation of Carotuximab, known as DE-122, was developed for intravitreal administration to treat persistent exudative (wet) age-related macular degeneration (AMD).[18] The rationale was based on evidence that endoglin is upregulated on choroidal vascular endothelial cells in AMD and may contribute to resistance to standard anti-VEGF therapies.[18] The Phase 1 PAVE study (NCT02555306) evaluated the safety of single ascending doses of intravitreal Carotuximab and found it to be generally well-tolerated.[18] However, despite this initial safety data, the collaboration with Santen Pharmaceuticals was later terminated following the drug's failure to demonstrate sufficient efficacy in this indication, leading to the discontinuation of its development for ophthalmologic diseases.[28]

Safety and Tolerability Profile

The safety profile of Carotuximab has been consistently characterized across its extensive clinical development program, involving various tumor types, combination agents, and dosing schedules. The overall profile is considered manageable, with most adverse events being low-grade and directly related to the drug's on-target inhibition of endoglin, a protein integral to vascular integrity.

Consolidated Safety Profile

Across numerous Phase 1 and 2 trials, Carotuximab has been shown to be well-tolerated both as a monotherapy and in combination with other anti-cancer agents.[18] In the recent Phase 2 trial in mCRPC (NCT05534646), the safety lead-in cohort of 10 patients treated with Carotuximab plus apalutamide reported no dose-limiting toxicities (DLTs), no unexpected adverse events (AEs), and no Grade 3 or 4 toxicities.[27] This favorable profile is consistent with earlier studies and suggests that Carotuximab does not substantially exacerbate the toxicity of its combination partners.

Mechanism-Related Adverse Events

The most frequently reported AEs are direct consequences of Carotuximab's biological mechanism of action—the inhibition of a protein essential for normal vascular development and homeostasis.[24] Because endoglin plays a role in maintaining the structural integrity of blood vessels, its inhibition can lead to predictable vascular-related side effects. These on-target effects include:

• Anemia: A dose-limiting hypoproliferative anemia was identified in the first-in-human Phase I study at a dose of 15 mg/kg weekly, which led to the establishment of the maximum tolerated dose (MTD) at 10 mg/kg weekly.[24] Anemia of a lesser grade is a common finding in subsequent trials.[29]
• Telangiectasias and Bleeding: Inhibition of endoglin can weaken small blood vessels, leading to the formation of telangiectasias (small, dilated blood vessels near the surface of the skin or mucous membranes) and an increased propensity for minor bleeding. Low-grade (Grade 1-2) epistaxis (nosebleeds) and gingival bleeding are among the most common AEs reported.[24]
• Infusion Reactions: As with many intravenously administered monoclonal antibodies, low-grade infusion-related reactions have been observed.[24]
• Headache and Fatigue: Headache and fatigue are also frequently reported AEs, although attribution can be confounded by the underlying cancer or the effects of combination agents.[29]

A consolidated summary of the most common adverse events associated with Carotuximab is provided in Table 3.

Table 3: Consolidated Profile of Common Adverse Events

Adverse Event	Common Grading	Frequency / Comments	Relevant Trial(s)
Anemia	Grade 1-2; DLT at high doses	Common. Dose-limiting toxicity (hypoproliferative anemia) observed at 15 mg/kg weekly.	24
Epistaxis (Nosebleed)	Grade 1-2	Very common. A predictable on-target effect of endoglin inhibition. Grade 3 was rare.	24
Gingival Bleeding	Grade 1-2	Common. Related to weakening of small vessels in the mucous membranes.	38
Telangiectasia	Grade 1-2	Common. Small, dilated blood vessels visible on skin/mucous membranes.	24
Headache	Grade 1-2	Very common. Often confounded by underlying disease or combination agents.	29
Fatigue	Grade 1-2	Very common. A nonspecific symptom frequent in cancer patients and with many therapies.	29
Infusion Reaction	Grade 1-2	Common. Typically manageable with standard supportive care.	24
Diarrhea / Nausea	Grade 1-2	Common. Often seen in combination with TKIs like pazopanib.	29

Contraindications and Potential Drug Interactions

A key contraindication for Carotuximab is a known diagnosis of Hereditary Hemorrhagic Telangiectasia (HHT), also known as Osler-Weber-Rendu syndrome.[36] HHT-1 is a rare genetic disorder caused by heterozygous loss-of-function mutations in the

ENG gene, which encodes endoglin.[38] Patients with HHT have fragile blood vessels and are at high risk for severe bleeding. Administering an endoglin-inhibiting antibody to these patients would be expected to exacerbate their condition, and they have therefore been consistently excluded from clinical trials.

Potential drug interactions are primarily pharmacodynamic. The risk of adverse effects may be increased when Carotuximab is combined with other monoclonal antibodies, particularly those with their own toxicity profiles (e.g., bevacizumab).[40] There is also a theoretical risk that estrogens may increase the thrombogenic activities of Carotuximab.[40] Additionally, as an immunosuppressive agent, Carotuximab could potentially decrease the therapeutic efficacy of live attenuated vaccines.[40]

Corporate and Regulatory Landscape

The development of Carotuximab has been characterized by a complex and evolving corporate and regulatory journey, involving multiple companies, strategic partnerships, and key interactions with regulatory agencies in the United States and Europe.

Developmental History: From TRACON Pharmaceuticals to Kairos Pharma

Carotuximab (as TRC-105) was originally developed by TRACON Pharmaceuticals, a San Diego-based biopharmaceutical company.[4] TRACON advanced the drug through extensive preclinical and early-to-mid-stage clinical development across a range of indications. The timeline of its corporate journey is marked by several key events that have shaped its trajectory [28]:

• March 2014: TRACON entered into a significant licensing agreement with Santen Pharmaceutical Co., Ltd., granting Santen rights to develop and commercialize the ophthalmic formulation of Carotuximab (DE-122) for diseases like wet AMD.
• April 2019: A major turning point occurred when the pivotal Phase 3 TAPPAS trial in angiosarcoma was terminated for futility, a significant setback that impacted the perceived value and future direction of the asset.
• March 2020: Following disappointing efficacy results in clinical studies for age-related macular degeneration, the licensing deal between TRACON and Santen was mutually terminated.
• May 2021: As part of a strategic shift, TRACON licensed Carotuximab to Enviro Therapeutics.
• July 2021: Kairos Pharma, Ltd. acquired Enviro Therapeutics in an all-stock transaction, thereby inheriting the global rights to Carotuximab and assuming responsibility for its continued development. This transfer has led to the recent focus on the mCRPC indication.

Regulatory Designations and Global Approval Status

Throughout its development, Carotuximab has received several important designations from regulatory bodies, which are intended to facilitate and expedite the development of drugs for serious conditions and/or rare diseases. These include:

• Orphan Drug Designation: Carotuximab was granted orphan drug designation by both the U.S. Food and Drug Administration (FDA) and the European Medicines Agency (EMA) for the treatment of soft tissue sarcoma.[1] This designation provides incentives such as market exclusivity and fee waivers to encourage the development of drugs for rare diseases.
• Fast Track Designation: In 2015, the FDA granted Fast Track designation to Carotuximab for the treatment of patients with advanced renal cell carcinoma.[1] This designation is designed to facilitate the development and expedite the review of drugs intended to treat serious conditions and fill an unmet medical need.

Despite these designations and an extensive clinical program, Carotuximab has not yet received marketing approval from any regulatory agency worldwide and remains an investigational therapeutic agent.[12] Its future regulatory path will be heavily dependent on the outcomes of its ongoing and planned clinical trials, most notably the Phase 2 study in mCRPC.

Synthesis and Future Outlook

The developmental saga of Carotuximab offers a compelling case study in the complexities, challenges, and strategic pivots inherent in modern oncology drug development. From its foundation on a robust scientific rationale to a history of mixed clinical results, the drug's trajectory has been anything but linear. Its future now stands at a critical inflection point, where recent promising data has the potential to redefine its therapeutic role and resurrect its clinical and commercial prospects.

Critical Analysis: Reconciling a Strong Preclinical Rationale with a Mixed Clinical History

The central paradox of Carotuximab is the disconnect between its well-characterized and compelling mechanism of action and its challenging clinical history, culminating in the failure of a pivotal Phase 3 trial. The preclinical evidence for targeting endoglin is strong: it is a validated driver of angiogenesis, its expression correlates with poor prognosis, and its inhibition shows clear anti-tumor effects in animal models. However, the clinical translation of this promise has been difficult.

The failure of the TAPPAS trial in angiosarcoma provides a crucial lesson. The initial strategy of positioning Carotuximab as a broad-spectrum anti-angiogenic agent to be added to existing standards of care proved flawed. In the presence of a potent, broad-acting VEGF pathway inhibitor like pazopanib, the additional, more targeted inhibition of the endoglin pathway offered no discernible clinical benefit. This suggests a level of mechanistic redundancy in certain contexts. The drug's true potential appears not to lie in general anti-angiogenesis, but in its ability to address specific, well-defined biological challenges where the endoglin pathway is a dominant and non-redundant driver of pathology.

The Promise in mCRPC: The Path Forward

The ongoing Phase 2 trial in metastatic castration-resistant prostate cancer represents the successful application of this lesson. The therapeutic hypothesis was refined: instead of targeting angiogenesis broadly, Carotuximab is being used to target a specific, acquired resistance mechanism—the upregulation of CD105 in response to AR signaling inhibition. The highly encouraging interim results, showing a median PFS of over 13 months in a heavily pre-treated population, provide the first strong clinical validation of this new hypothesis.

This result has fundamentally altered the narrative for Carotuximab. It is no longer a "failed" anti-angiogenic drug but a promising "resistance-reversal" agent. The path forward for the asset is now clear and focused: the successful completion of the current Phase 2 study is paramount. Positive final results will provide the foundation for discussions with regulatory agencies and the design of a pivotal Phase 3 trial in this indication. A successful outcome in mCRPC would not only establish a new standard of care for a patient population with limited options but would also validate the targeting of CD105 as a viable strategy to overcome resistance to AR-targeted therapies.

Future Research Directions and Key Unanswered Questions

While the focus is rightly on mCRPC, the complex biology of Carotuximab suggests several other avenues for future research. Key unanswered questions remain that could further unlock its potential:

• Biomarker Development: Can a predictive biomarker be identified to select patients most likely to benefit from Carotuximab? Investigating baseline tumor or serum levels of CD105, soluble endoglin, BMP9, or other components of the TGF-β pathway could be critical for optimizing its use.
• Combination with Immunotherapy: Given Carotuximab's demonstrated immunomodulatory effects, including the potential to deplete immunosuppressive Tregs, combining it with immune checkpoint inhibitors (e.g., anti-PD-1/PD-L1 antibodies) is a highly compelling strategy that warrants preclinical and clinical investigation.
• Exploration in Other Resistance Settings: The success in overcoming resistance to AR inhibitors in prostate cancer raises the question of whether Carotuximab could reverse resistance to other targeted therapies or chemotherapies where endoglin upregulation is a known escape mechanism. For instance, recent research has suggested a role for CD105 in mediating resistance to EGFR inhibitors in non-small cell lung cancer, offering another potential application.[43]

In conclusion, Carotuximab is a drug that has been redefined by its clinical journey. Its story highlights that in an era of targeted therapy, a deep understanding of mechanism and a willingness to pivot strategy in response to clinical data are essential for success. Once facing an uncertain future after a late-stage failure, Carotuximab has been repositioned as a potentially transformative therapy for advanced prostate cancer, illustrating the remarkable resilience and dynamism of oncologic drug development.

Works cited

1. Carotuximab Overview - Creative Biolabs, accessed September 28, 2025, https://www.creativebiolabs.net/carotuximab-overview.htm
2. Carotuximab | Buy from Supplier AdooQ®, accessed September 28, 2025, https://www.adooq.com/carotuximab.html
3. Definition of carotuximab - NCI Drug Dictionary, accessed September 28, 2025, https://www.cancer.gov/publications/dictionaries/cancer-drug/def/carotuximab
4. Carotuximab - Wikipedia, accessed September 28, 2025, https://en.wikipedia.org/wiki/Carotuximab
5. A Simple Methodology for Conversion of Mouse Monoclonal ..., accessed September 28, 2025, https://pmc.ncbi.nlm.nih.gov/articles/PMC3775440/
6. Carotuximab (Anti-Endoglin / CD105) CD markers antibody | Read Reviews & Product Use Citations - Selleck Chemicals, accessed September 28, 2025, https://www.selleckchem.com/products/carotuximab-anti-endoglin-cd105.html
7. Production and purification of a chimeric monoclonal antibody against botulinum neurotoxin serotype A - UNL Digital Commons, accessed September 28, 2025, https://digitalcommons.unl.edu/cgi/viewcontent.cgi?article=1033&context=chemeng_biotechnology
8. Monoclonal Antibody Manufacturing Processes - Avantor, accessed September 28, 2025, https://www.avantorsciences.com/us/en/support/knowledge-center/monoclonal-antibody-production-process
9. RCSB PDB: Homepage, accessed September 28, 2025, https://www.rcsb.org/
10. 3PP3: Epitope characterization and crystal structure of GA101 provide insights into the molecular basis for the type I / type II distinction of anti- CD20 antibodies - RCSB PDB, accessed September 28, 2025, https://www.rcsb.org/structure/3pp3
11. 4KAQ: Structure of rituximab Fab - RCSB PDB, accessed September 28, 2025, https://www.rcsb.org/structure/4kaq
12. Carotuximab : Drug Detail - Cancer Knowledgebase (CKB), accessed September 28, 2025, https://ckb-core.genomenon.com/drug/show/7227
13. carotuximab | Ligand page | IUPHAR/BPS Guide to MALARIA PHARMACOLOGY, accessed September 28, 2025, https://test.guidetomalariapharmacology.org/GRAC/LigandDisplayForward?tab=summary&ligandId=9098
14. Carotuximab | MedPath, accessed September 28, 2025, https://trial.medpath.com/drug/fdaabb2a0e37124c/carotuximab
15. Anti-Endoglin/CD105 Antibody (Carotuximab), accessed September 28, 2025, https://www.apexbt.com/downloader/document/F2034/Datasheet.pdf
16. Monoclonal anti-endoglin antibody TRC105 (carotuximab) prevents hypercholesterolemia and hyperglycemia-induced endothelial dysfunction in human aortic endothelial cells, accessed September 28, 2025, https://pmc.ncbi.nlm.nih.gov/articles/PMC9490272/
17. Endoglin Targeting: Lessons Learned and Questions That Remain - MDPI, accessed September 28, 2025, https://www.mdpi.com/1422-0067/22/1/147
18. Safety and Tolerability of Intravitreal Carotuximab (DE-122) in Patients With Persistent Exudative Age-Related Macular Degeneration: A Phase I Study - PMC, accessed September 28, 2025, https://pmc.ncbi.nlm.nih.gov/articles/PMC8711001/
19. Carotuximab (TRC105) | Anti-Endoglin mAb - MedchemExpress.com, accessed September 28, 2025, https://www.medchemexpress.com/carotuximab.html
20. Carotuximab (TRC105; DE-122) | CAS 1268714-50-6 | AbMole, accessed September 28, 2025, https://www.abmole.com/products/carotuximab.html
21. (PDF) Monoclonal anti-endoglin antibody TRC105 (carotuximab) prevents hypercholesterolemia and hyperglycemia-induced endothelial dysfunction in human aortic endothelial cells - ResearchGate, accessed September 28, 2025, https://www.researchgate.net/publication/363374298_Monoclonal_anti-endoglin_antibody_TRC105_carotuximab_prevents_hypercholesterolemia_and_hyperglycemia-induced_endothelial_dysfunction_in_human_aortic_endothelial_cells
22. pmc.ncbi.nlm.nih.gov, accessed September 28, 2025, https://pmc.ncbi.nlm.nih.gov/articles/PMC8711001/#:~:text=Carotuximab%20inhibits%20VEGF%2Dinduced%20and,and%20promotion%20of%20VEGFR2%20degradation.
23. Endoglin inhibitor TRC105 with or without bevacizumab for bevacizumab-refractory glioblastoma (ENDOT): a multicenter phase II trial - PMC - PubMed Central, accessed September 28, 2025, https://pmc.ncbi.nlm.nih.gov/articles/PMC10491825/
24. A Phase I First-In-Human Study of TRC105 (Anti-Endoglin Antibody) in Patients With Advanced Cancer - PubMed, accessed September 28, 2025, https://pubmed.ncbi.nlm.nih.gov/22767667/
25. Pharmacokinetics. A, Mean peak TRC105 serum concentrations. B, Dose... - ResearchGate, accessed September 28, 2025, https://www.researchgate.net/figure/Pharmacokinetics-A-Mean-peak-TRC105-serum-concentrations-B-Dose-proportional_fig4_316649406
26. Cetuximab pharmacokinetics influences progression-free survival of metastatic colorectal cancer patients - PubMed, accessed September 28, 2025, https://pubmed.ncbi.nlm.nih.gov/21953502/
27. Carotuximab/Apalutamide Improves PFS in Previously Treated Metastatic CRPC, accessed September 28, 2025, https://www.cancernetwork.com/view/carotuximab-apalutamide-improves-pfs-in-previously-treated-metastatic-crpc
28. Kairos sets its prostate cancer expectations | ApexOnco - Clinical ..., accessed September 28, 2025, https://www.oncologypipeline.com/apexonco/kairos-sets-its-prostate-cancer-expectations
29. Efficacy and Safety of TRC105 Plus Pazopanib vs Pazopanib Alone for Treatment of Patients With Advanced Angiosarcoma: A Randomized Clinical Trial - ResearchGate, accessed September 28, 2025, https://www.researchgate.net/publication/359638286_Efficacy_and_Safety_of_TRC105_Plus_Pazopanib_vs_Pazopanib_Alone_for_Treatment_of_Patients_With_Advanced_Angiosarcoma_A_Randomized_Clinical_Trial
30. Compound: CAROTUXIMAB (CHEMBL2109321) - ChEMBL - EMBL-EBI, accessed September 28, 2025, https://www.ebi.ac.uk/chembl/explore/compound/CHEMBL2109321
31. A phase 1b dose-escalation study of TRC105 (anti-endoglin antibody) in combination with axitinib in patients with metastatic renal cell carcinoma (mRCC). - ASCO Publications, accessed September 28, 2025, https://ascopubs.org/doi/10.1200/jco.2014.32.15_suppl.e15562
32. Carotuximab Completed Phase 1 Trials for Non-Small Cell Lung Cancer (NSCLC) Treatment | DrugBank Online, accessed September 28, 2025, https://go.drugbank.com/drugs/DB06322/clinical_trials?conditions=DBCOND0034130&phase=1&purpose=treatment&status=completed
33. Advanced Soft Tissue Sarcoma Completed Phase 1 / 2 Trials for Carotuximab (DB06322), accessed September 28, 2025, https://go.drugbank.com/indications/DBCOND0060596/clinical_trials/DB06322?phase=1%2C2&status=completed
34. (PDF) Detection of endoglin-expressing CTCs in patients enrolled in an adaptive enrichment phase 3 trial of TRC105 and pazopanib versus pazopanib alone in patients with advanced angiosarcoma (TAPPAS). - ResearchGate, accessed September 28, 2025, https://www.researchgate.net/publication/327456662_Detection_of_endoglin-expressing_CTCs_in_patients_enrolled_in_an_adaptive_enrichment_phase_3_trial_of_TRC105_and_pazopanib_versus_pazopanib_alone_in_patients_with_advanced_angiosarcoma_TAPPAS
35. A randomized phase II study of apalutamide with or without carotuximab (ant-CD105) in metastatic, castration-resistant prostate cancer. - ASCO Publications, accessed September 28, 2025, https://ascopubs.org/doi/10.1200/JCO.2023.41.16_suppl.TPS5107
36. Carotuximab/Apalutamide Shows Tolerability in Metastatic CRPC - Cancer Network, accessed September 28, 2025, https://www.cancernetwork.com/view/carotuximab-apalutamide-shows-tolerability-in-metastatic-crpc
37. Carotuximab plus apalutamide demonstrates safety, tolerability in mCRPC - Urology Times, accessed September 28, 2025, https://www.urologytimes.com/view/carotuximab-plus-apalutamide-demonstrates-safety-tolerability-in-mcrpc
38. A Phase 1 First-in-Human Study of TRC105 (Anti-Endoglin Antibody ..., accessed September 28, 2025, https://pmc.ncbi.nlm.nih.gov/articles/PMC3432706/
39. Endoglin inhibitor TRC105 with or without bevacizumab for ..., accessed September 28, 2025, https://www.ncbi.nlm.nih.gov/pmc/articles/PMC10491825/
40. Carotuximab: Uses, Interactions, Mechanism of Action | DrugBank Online, accessed September 28, 2025, https://go.drugbank.com/drugs/DB06322
41. TRACON Pharmaceuticals Announces Successful Meetings With FDA And EMA For TRC105 (carotuximab) In Angiosarcoma - Clinical Leader, accessed September 28, 2025, https://www.clinicalleader.com/doc/tracon-pharmaceuticals-announces-successful-carotuximab-angiosarcoma-0001
42. Enviro Therapeutics - Carotuximab - AdisInsight - Springer, accessed September 28, 2025, https://adisinsight.springer.com/drugs/800027728
43. Carotuximab - Drug Targets, Indications, Patents - Patsnap Synapse, accessed September 28, 2025, https://synapse.patsnap.com/drug/70af8ac8c4424fe7896fe8e8c4cba278