A phase I study of obatoclax mesylate, a Bcl-2 antagonist, plus topotecan in solid tumor malignancies

To establish the safety, maximum tolerated dose (MTD), recommended phase II dose, and preliminary antitumor activity of obatoclax mesylate (GX15-070MS), a Bcl-2 antagonist, in combination with topotecan in patients with solid tumor malignancies.

Patients with solid tumor malignancies for whom topotecan was an appropriate treatment were administered obatoclax mesylate and topotecan on a 3-week cycle in a pre-defined, standard 3 + 3 dose escalation scheme. The starting dose for obatoclax mesylate was 14 mg/m² by 3-h intravenous (IV) infusion. Topotecan 1.25 mg/m² was given concurrently as an IV infusion on days 1–5 of each cycle.

Fourteen patients received 40 cycles of obatoclax mesylate at the following doses: 14 mg/m² on day 1, 14 mg/m² on days 1 and 3, and 20 mg/m² on day 1. The most common toxicities related to obatoclax were neurologic, including ataxia, mood alterations, somnolence, and cognitive dysfunction. The majority of these were grades 1 and 2 (88%). Two of five patients experienced dose-limiting grade 3 neurologic toxicity at a dose of 20 mg/m²; no patients experienced grade 4 neurologic toxicities, and no other patients experienced grade 3 neurologic toxicity. Of the patients who experienced grade 3 neurologic events, one later developed febrile neutropenia, which was also a dose-limiting toxicity (DLT). After an additional three patients were treated without DLT at the previously tolerated dose of 14 mg/m² on day 1, the level was escalated to 14 mg/m² on days 1 and 3. Three patients were treated at this dose and, with none experiencing a DLT, 14 mg/m² on days 1 and 3 was defined as the recommended phase II dose. Two patients with small-cell lung cancer (SCLC) achieved partial responses and four patients had stable disease. Median time to progression (TTP) was 12 weeks.

Obatoclax mesylate administered at 14 mg/m² IV on days 1 and 3 is safe and well tolerated when given in combination with topotecan 1.25 mg/m² IV on days 1–5 of an every 3-week cycle. A phase II trial to assess the efficacy of this combination for patients with relapsed SCLC is currently accruing patients.