CAL-101, An Isoform-Selective Inhibitor of Phosphatidylinositol 3-Kinase P110δ, Demonstrates Clinical Activity and Pharmacodynamic Effects In Patients with Relapsed or Refractory Chronic Lymphocytic Leukemia

The class I phosphatidylinositol 3-kinases (PI3Ks) regulate cellular functions relevant to oncogenesis. Expression of the PI3K p110δ isoform (PI3Kδ) is restricted to cells of hematopoietic origin where it plays a key role in B cell proliferation and survival. In chronic lymphocytic leukemia (CLL) the PI3K pathway is constitutively activated and dependent on PI3Kδ. CAL-101 is an isoform-selective inhibitor of PI3Kδ (EC₅₀ of 62 nM in a whole-blood assay with >200-fold selectivity relative to other PI3K isoforms) that inhibits PI3K signaling and induces apoptosis of CLL cells in vitro.

This Phase 1 study evaluated the safety, pharmacokinetics, pharmacodynamics and clinical activity of CAL-101 in patients with relapsed or refractory hematologic malignancies. Sequential cohorts of patients were enrolled at progressively higher dose levels with cohort expansion based on toxicity profile and plasma exposure. CAL-101 was administered orally one or 2 times per day (QD or BID) continuously for 28-day cycles for up to 12 cycles (with the potential for more prolonged therapy on an extension protocol thereafter). Clinical response was evaluated according to standard criteria.

At data cutoff, the study had enrolled 37 patients with CLL. Patients included: males/females n=31 (84%)/6 (16%) with median age of 65 [range 37–82] years, refractory/relapsed disease n=24 (65%)/13 (35%), bulky disease n= 29 (81%), and adverse cytogenetics of del(17p), del(11q) or both n=22 (63%). The median number of prior therapies was 5 [range 2–14]. The number (%) of patients with specific prior therapies included: rituximab n=37 (100%), purine analog n=37 (100%), alkylating agent n= 31 (84%), and alemtuzumab n=12 (32%). CAL-101 dose levels were 50 mg BID (n=1), 100 mg BID (n=4), 150 mg BID (n=11), 200 mg BID (n=10), 350 mg BID (n=7) and 300 mg QD (n=4). The median number of treatment cycles was 9 [range 1–13], with 21 (57%) patients continuing on treatment (11 on study and 10 on the extension protocol after 12 cycles). Symptomatic adverse events were infrequent, usually low-grade, and not clearly CAL-101-related. Grade ≥3 pneumonias occurred in 9 (24%) patients. Grade ≥3 hematological laboratory abnormalities included neutropenia n=9 (24%), thrombocytopenia n=4 (11%) and anemia n=3 (8%) that were not usually considered CAL-101-related. A pharmacokinetic analysis of dose-proportionality showed minimal increases in plasma C_max and AUC at CAL-101 doses >150 mg BID; these data, taken together with the tumor regression results, have proved helpful in supporting Phase 2–3 dose selection. Flow cytometry of CLL cells from patients showed that CAL-101 reduced constitutive expression of phospho-AKT to background levels when measured after 1 week of treatment (p<0.0001), demonstrating pharmacodynamic inhibition of activated PI3K signaling. Plasma concentrations of chemokines CCL3, CCL4, and CXCL13 were elevated at baseline and decreased significantly within 1 cycle of CAL-101 administration (p<0.001 for all comparisons). CAL-101 reduced lymphadenopathy in all 32 (100%) patients with at least 1 post-treatment tumor assessment; 29/32 (91%) achieved a lymph node response (≥50% reduction in target nodal lesions). An initial increase in peripheral absolute lymphocyte counts of >50% from baseline was observed in 21/35 (60%) patients; increases were maximal during the first 2 cycles and decreased thereafter; the pattern suggested drug-mediated lymphocyte redistribution. Considering nodal and peripheral blood changes together, partial responses were observed in 11/33 (33%) of patients. The median duration of response had not been reached; 7 patients had response durations of ≥6 months. Of 20 patients with CLL-related thrombocytopenia (baseline platelet counts <100,000/μ L), 15 (75%) had either an improvement to >100,000/μ L or a >50% increase from baseline.

CAL-101, an oral PI3Kδ isoform-selective inhibitor, shows acceptable toxicity, positive pharmacodynamic effects, and favorable clinical activity in heavily pretreated patients with CLL, including patients with refractory disease, bulky lymphadenopathy, and poor-prognosis cytogenetics. The high level of lymph node regression and prolonged duration of symptomatic tumor control strongly support evaluation of CAL-101 alone and in combination with other chemo/immunotherapy approaches to CLL management.

Byrd:Calistoga Pharmaceuticals: Consultancy, Equity Ownership. Brown:Calistoga: Consultancy. Kahl:Calistoga Pharmaceuticals: Consultancy, Research Funding. Lannutti:Calistoga Pharmaceutical Inc.: Employment. Giese:Calistoga Pharmaceuticals: Equity Ownership. Webb:Calistoga Pharmaceuticals: Employment. Ulrich:Calistoga Pharmaceuticals: Employment, Equity Ownership. Peterman:Calistoga Pharmaceuticals: Employment. Holes:Calistoga Pharmaceuticals: Employment. Yu:Calistoga Pharmaceuticals: Employment, Equity Ownership.