The U.S. Food and Drug Administration (FDA) has granted Fast Track designation to ARX517, a proprietary anti-PSMA antibody-drug conjugate (ADC) investigational being therapy developed by Ambrx for the treatment of patients with metastatic castration-resistant prostate cancer (mCRPC) upon progression on an androgen receptor pathway inhibitor.

Fast Track designation is a process designed to facilitate the development and expedite the review of drugs which may demonstrate substantial improvement over available therapy for serious conditions with unmet medical need.

“Receiving Fast Track designation from the FDA reinforces Ambrx’s belief in ARX517 as a potential novel treatment for mCRPC and underscores the urgent need for improved treatment options for patients at this advanced stage of prostate cancer,” said Sandra Aung, Ph.D., Chief Clinical Officer of Ambrx.

First-in-human
ARX517 is currently being studied in APEX-01 (ClinicalTrials.gov ID NCT04662580), a Phase 1/2, first-in-human, open label dose escalation and dose expansion trial enrolling patients with mCRPC whose tumors have progressed on at least two prior FDA approved treatments for prostate cancer, and patients must have one of the following three criteria met:

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  • PSA progression defined by a minimum of 2 rising PSA values; or
  • Radiographic progression by RECIST v 1.1, or
  • Disease progression by the presence of new bone lesions.

The APEX-01 trial is the only ongoing clinical trial in the United States targeting PSMA with an ADC.

“This Fast Track designation represents a significant milestone for Ambrx. Enabled by our expanded genetic code site-specific conjugation technology, ARX517 has the potential to be a more effective and tolerable treatment option for these patients, and we are planning to present updated preliminary data from APEX-01 at a major medical meeting this fall,” noted Daniel J. O’Connor, Chief Executive Officer of Ambrx.

ARX517 is an antibody-drug conjugate composed of a fully humanized anti-PSMA mAb linked to AS269, a proprietary and potent microtubule inhibitor. Prostate-specific membrane antigen (PSMA) is highly expressed (>89%) in mCRPC and has been shown to be a validated therapeutic target. Upon binding to PSMA on the surface of cancer cells, ARX517 is internalized and pAF-AS269, its cancer cell killing payload, is released following lysosomal metabolism.

ARX517’s site-specific conjugation, non-cleavable linker and stable linker chemistry is achieved by first incorporating the nonnatural amino acid, para-acetyl phenylalanine (pAF), into the antibody, followed by covalent conjugation of AS269 to the pAF to form a highly stable oxime bond, enabled by Ambrx’s proprietary expanded genetic code technology which was recently featured in Nature.[1]

ARX517’s linker has exceptional stability, and the resulting ADC exhibits a homogenous drug-to-antibody ratio and mAb-like biophysical properties. Therefore, it is believed that ARX517 may have the potential to promote highly specific tumor cell killing with minimal off-target toxicity. ARX517 has the potential to be a first- and best-in-class anti-PSMA ADC addressing the high unmet medical need in mCRPC.

Clinical trials
ARX517 in Subjects With Metastatic Castration-resistant Prostate Cancer (ARX517) – NCT04662580.

ADC Drugmap
ARX517

Reference
[1] Mair MJ, Bartsch R, Le Rhun E, Berghoff AS, Brastianos PK, Cortes J, Gan HK, Lin NU, Lassman AB, Wen PY, Weller M, van den Bent M, Preusser M. Understanding the activity of antibody-drug conjugates in primary and secondary brain tumours. Nat Rev Clin Oncol. 2023 Jun;20(6):372-389. doi: 10.1038/s41571-023-00756-z. Epub 2023 Apr 21. PMID: 37085569.

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