A Phase I/II Study to Assess the Safety and Tolerability of APO866 for the Treatment of Refractory B-CLL

B-CLL is one of the most common types of leukemia, remains incurable, and has only limited therapeutic options available including alkylating agents and fludarabine. The identification of new, selective, and non-toxic forms of therapy for patients with B-CLL is therefore a high priority.

APO866 is a novel drug that induces cell death by specifically inhibiting the biosynthesis of NAD+ from niacinamide, which is essential for the cellular metabolism, protein modification and messenger synthesis. APO866 is not subject to the commonly known mechanisms of MDR. Its activity is cell cycle independent. APO866 exerted high anti-tumor activity on a broad range of different tumor cells derived from both human solid cancers and leukemias in vitro, including CD38- and CD38+ cell lines, and on a large number of human xenografts in nude mice and rats in vivo. Hematologic cancer cells were highly sensitive to APO866 (lower than 6 nM). Lymphocytes are the most sensitive normal cells to APO866 resulting in lymphocytopenia and reticulocytopenia in rats and monkeys. Furthermore, APO866 may have anti-angiogenic properties as shown in vivo and dose-dependent decrease of VEGF levels in patients treated in the phase I study.

It is unclear why CD38 positivity is associated with a short patient survival and poor responsiveness to chemotherapy. It is possible that CD38 expression confers to B-CLL of a more malignant cellular phenotype. This seems plausible given that the antigen has an important role as a modulator of intracellular signaling and that cross-linking of CD38 up-regulates BCL-2 and inhibits apoptosis in normal mature B cells. Indeed, CD38/CD3l interactions lead to increased B-CLL proliferation and survival. This study has been designed to recruit patients with progressive or refractory B-CLL of whom, in line with the literature, about 50% will be CD38+.

We hypothesize that CD38+ lymphocytes will be more sensitive to the APO866 therapy due to increased intracellular NAD+ consumption as substrate for the PARP dependent DNA repair (instable genome, proliferation). However, CD38+ and CD38- B-CLL cells seem to be equally sensitive to APO866 in vitro. In addition, decreased intracellular NAD+ levels will reduce the anabolism of cADPR. CD38+ can catalyze the synthesis of cADPR from NAD+ and can further hydrolyze cADPR to ADP-ribose.85,86,87 The cADPR is a potent Ca++-mobilizing activator and a potential endogenous regulator.88 The ability of cADPR to release Ca++ from intracellular pools is thought to be part of CD38-mediated signalling pathways that result in cell growth, apoptosis, and differentiation.

APO866 was investigated in 24 patients with advanced cancers in a phase I study aiming to determine the DLT and MTD. Treatment was well tolerated and safe. The unique DLT was thrombocytopenia. At dose levels higher than 0.036 mg/m2/hr CTC grade III lymphocytopenia, not thought to be clinically relevant, preceded all other toxicities. The recommended dose for phase II studies of APO866 is 0.126 mg/m2/hr administered by civ infusion for 4 consecutive days every 4 weeks. This dose was selected because of its safety profile, and the translational observation that Css of APO866 at MTD was similar or higher as compared to the concentrations at which efficacy was established in vitro and in vivo. In addition, a transient decrease of serum VEGF levels, a surrogate marker of angiogenesis, was observed within 96 hrs after the start of treatment in 9 out of 11 patients treated at MTD and the 0.144 mg/m2/hr dose level of APO866

This forms the rationale to conduct a "Proof of concept" study of APO866, administered at the recommended dose in patients with progressive refractory B-CLL non-eligible for aHSCT, either CD38- or CD38+.