ASCO 2021: First-in-Human Study of TAS3681, an Oral Androgen Receptor Antagonist with AR and AR Splice Variant Downregulation Activity, in Patients with mCRPC Refractory to Abiraterone And/or Enzalutamide and Chemotherapy

(UroToday.com) Second-generation androgen receptor signaling inhibitors have improved outcomes for men with metastatic castration-resistant prostate cancer (mCRPC), however, drug resistance invariably evolves with androgen receptor overexpression, androgen receptor mutation, or androgen receptor splice variants and subsequently continued androgen receptor signaling. TAS3681 is an oral and selective androgen receptor antagonist with androgen receptor and androgen receptor-splice variant down-regulation. This molecule also suppresses androgen-independent androgen receptor transactivation and has antitumor efficacy in androgen receptor-splice variant positive, enzalutamide-resistant CRPC models. As follows is a schematic of the TAS3681 activity in the androgen receptor pathway:

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At the 2021 American Society of Clinical Oncology (ASCO) Annual Meeting, Dr. Johann de Bono and colleagues presented the dose-escalation part of the first-in-human trial of TAS3681 in patients with mCRPC.

For this trial, mCRPC patients with progressing disease after abiraterone and/or enzalutamide, and ≥1 additional chemotherapy, received TAS3681 in a 3+3 dose escalation design. QD or BID tablets were given in 28-day cycles and BID dosing at ≤600 mg was introduced to increase daily exposure while limiting Cmax. The primary endpoints for this trial included the incidence of dose-limiting toxicities and adverse events. Other endpoints included pharmacokinetics and antitumor activity per Prostate Cancer Clinical Trials Working Group 3 (PCWG3).

As of January 22, 2021, 56 pts in 10 cohorts were dosed (QD: 25, 50, 100, 200, 400, 600, 800, and 1000 mg; and BID: 300 and 400 mg):

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The median age was 66 (56–79) years and patients had a median of 6 prior lines of systemic therapy. Of 41 patients evaluable for dose-limiting toxicities, 3 had confirmed dose-limiting toxicities: 1/10 evaluable patients at 600 mg QD (QT prolongation >480 ms) and 2/3 evaluable patients at 400 mg BID (1 patient with QT prolongation >480 ms, 1 patient with grade 3 hypertension). The most common treatment-related adverse events were: nausea (32 patients, 57.1%), hyperbilirubinemia (21 patients, 37.5%), fatigue (18 patients, 32.1%), vomiting (17 patients, 30.4%) and diarrhea (16 patients, 28.6%). Adverse events of QT prolongation were seen in 11 patients (19.6%), and treatment-related adverse events ≥ in grade 3 were reported in 12 patients (21.4%). TAS3681 exposure increased dose-dependently up to 600 mg QD and then plateaued. Steady-state was reached by Day 8 and accumulation ratios of AUC were approximately 2 to 6 times. Confirmed PSA declines of >50% from baseline were seen in the 600 mg QD (2 patients) and 300 mg BID cohorts (1 patient):

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Antitumor activity was observed, with a tumor response rate of 23.1% in the 600 mg QD cohort (3 confirmed partial responses in 13 dosed patients), 22.2% in the 300 mg BID cohort (1 confirmed partial response and 1 unconfirmed complete response in 9 dosed patients), and 14.3% in the 400 mg BID cohort (confirmed partial response in 1/7 dosed patients). Responses occurred after 2–4 cycles of treatment. The longest duration of response to date is 16.2 months, and the 5 patients with confirmed partial response had a duration of response > 6 months with the exception of 1 patient who had a short follow-up period:

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300 mg BID was found to be the best-tolerated dose with antitumor activity based on: (i) median treatment duration was longer in 300 mg BID than in 600 mg QD, (ii) daily exposure AUC was comparable between 300 mg BID and 600 mg QD; Cmax was lower in the BID cohort, (iii) both cohorts showed similar efficacy in terms of PSA response and tumor response, and (iv) 300 mg BID showed a slightly better safety profile than 600 mg QD.

Dr. de Bono concluded his presentation with the following take-home messages:

  • TAS3681 is an oral androgen receptor full-length and androgen receptor-splice variant antagonist, which has a manageable safety profile and has antitumor activity against heavily pretreated, multi-drug resistant mCRPC at the recommended phase 2 dose of 300 mg BID
  • The study expansion phase is enrolling patients who have progressed on abiraterone or enzalutamide +/- taxane chemotherapy

Clinical trial information: NCT02566772

Presented By: Johann de Bono, MD, MSc, Ph.D., FRCP, FMedSci, Institute of Cancer Research and Royal Marsden NHS Foundation Trust, Sutton, Surrey, United Kingdom

Written By: Zachary Klaassen, MD, MSc, Urologic Oncologist, Assistant Professor of Urology, Georgia Cancer Center, Augusta University/Medical College of Georgia Twitter: @zklaassen_md at the 2021 American Society of Clinical Oncology (ASCO) Annual Meeting, Virtual Annual Meeting #ASCO21, June, 4-8, 2021

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