Substantial susceptibility of chronic lymphocytic leukemia to BCL2 inhibition: results of a phase I study of navitoclax in patients with relapsed or refractory disease

Abstract

Purpose: BCL2 overexpression is a hallmark of chronic lymphocytic leukemia (CLL). The novel BH3 mimetic navitoclax (ABT-263) specifically inhibits BCL2 and related proteins BCL-x(l) and BCL-w, potently inducing apoptosis of CLL cells in vitro. A phase I trial in patients with CLL was conducted to evaluate the safety, pharmacokinetics, and biologic activity of oral navitoclax.

Patients and methods: Twenty-nine patients with relapsed or refractory CLL received daily navitoclax for 14 days (10, 110, 200, or 250 mg/d; n = 15) or 21 days (125, 200, 250, or 300 mg/d; n = 14) of each 21-day cycle. Dose escalation decisions were informed by continual reassessment methodology.

Results: Lymphocytosis was reduced by more than 50% in 19 of 21 patients with baseline lymphocytosis. Among 26 patients treated with navitoclax ≥ 110 mg/d, nine (35%) achieved a partial response and seven maintained stable disease for more than 6 months. Median treatment duration was 7 months (range, 1 to ≥ 29 months). Median progression-free survival was 25 months. Activity was observed in patients with fludarabine-refractory disease, bulky adenopathy, and del(17p) CLL. Thrombocytopenia due to BCL-x(l) inhibition was the major dose-limiting toxicity and was dose-related. Low MCL1 expression and high BIM:MCL1 or BIM:BCL2 ratios in leukemic cells correlated with response. We determined that the navitoclax dose of 250 mg/d in a continuous dosing schedule was optimal for phase II studies.

Conclusion: BCL2 is a valid therapeutic target in CLL, and its inhibition by navitoclax warrants further evaluation as monotherapy and in combination in this disease.

Fig 1.

Patient status on study by dose and response. For the intermittent schedule cohort (A) and the continuous schedule cohort (B), the upper horizontal bar graphs indicate the time on study for each patient and the lower tables present a summary of dose-limiting toxicities (DLTs) and dose modifications. For the time on study graph, patients are grouped according to their observed best response, and the color of the bar reflects the navitoclax dose cohort they entered. For patients achieving a partial response (PR), the time on study to reach PR is represented by the lighter blue segment of the bar. When a patient discontinued the study, the reason is indicated on the right end of the colored bar, and for patients still receiving navitoclax, their duration on study in months is similarly indicated. AE, adverse event; De, death; ID, investigator discretion; PD, progressive disease; WC, withdrew consent.

Fig 2.

Navitoclax reduces platelet counts and chronic lymphocytic leukemia (CLL) burden in peripheral blood (PB) and lymph nodes. (A) Navitoclax scheduling modulates the severity of thrombocytopenia. The platelet counts of patients receiving 250 mg navitoclax daily in either the 14-days-on/7-days-off intermittent dosing cohort or the continuous dosing cohort incorporating a 7-day 100-mg lead-in phase are plotted for the first 50 days. The dotted line represents a platelet count of 25 × 10⁹/L, denoting the grade 4 toxicity level. One patient in the continuous cohort interrupted dosing prematurely for nonthrombocytopenic toxicity. (B) Navitoclax pharmacokinetics: The mean ± standard deviation plasma concentrations of navitoclax are plotted for the hours following initial dosing on day 1 and on day 14 for each dose cohort on the intermittent dosing schedule. (*) On day 14, n = 3 for 110 mg, n = 4 for 200 mg, and n = 2 for 250 mg. (C) Waterfall plot of most favorable changes in PB lymphocytosis. Only patients with lymphocytosis (> 5 × 10⁹/L) at study entry are included. (D) Rapid reduction in PB lymphocytosis. The PB lymphocyte counts (× 10⁹/L) during cycles 1 to 3 for individual patients enrolled in the 250-mg dose cohorts on either the intermittent or continuous schedules are presented. (E) Navitoclax induces apoptosis of CLL cells in vivo. Representative immunoblots of lysates of PB CLL cells from a patient from each of the 10-mg, 200-mg (intermittent), and 300-mg (continuous) dosing cohorts. For each patient, lysates from screening (Scr; pre-exposure), day 14 of cycle 1 (C1), and day 1 of cycle 3 (C3) were electrophoresed, before membranes were probed for caspase 3, cleaved caspase 3, and glyceraldehyde 3-phosphate dehydrogenase (GAPDH). In the 200-mg and 300-mg examples, but not the 10-mg example, the presence of cleaved caspase 3 in the cycle 1 and cycle 3 lanes reflects activation of apoptosis during exposure to navitoclax. GAPDH is reduced in these particular samples because it is known to be degraded during apoptosis. Cleaved caspase 3 was detected in six of nine patients analyzed after receiving ≥ 110 mg/d of navitoclax. (F) Waterfall plot of most favorable change in lymphadenopathy. Asterisks identify patients with bulky lymphadenopathy (> 5 cm diameter masses) at study entry. (G) Representative images from abdominal computed tomography scans of a patient with fludarabine-refractory del(17p) CLL at study entry (left) and after seven cycles of navitoclax 200 mg/d (right). Bulky mesenteric adenopathy has almost completely resolved. A benign left renal cyst has not altered over time. This response has been maintained for more than 2 years.

Fig 3.

Durability of antileukemic activity of navitoclax. (A) The progression-free survival (PFS) for all patients receiving ≥ 110 mg/d navitoclax from study entry (n = 26) is displayed in a Kaplan-Meier plot. (B) PFS for a subset of these patients with fludarabine-refractory disease. Fluorescent in situ hybridization data were available for 22 of 26 patients receiving ≥ 110 mg/d navitoclax. (C) PFS for patients with del17p13.2 chronic lymphocytic leukemia (n = 9), either del17p13.2 or del11p22.3 (n = 16), or chronic lymphocytic leukemia with neither of these abnormalities (n = 6). NA, not applicable.

Fig 4.

High-level BCL2 expression, but not BCL-w or BCL-x_l, in peripheral blood chronic lymphocytic leukemia cells and inverse correlation of MCL1 expression with response. The concentrations of BCL2, BCL-w, BCL-x_l, BIM, and MCL1 were measured as described in Patients and Methods by using immunoblots. BCL-x_l was typically not detected (< 6 fmol/10⁶ cells). (A) Concentrations of BCL2 and BCL-w for individual patients treated at two centers. (B and C) BCL2 concentrations and BIM:BCL2 ratios determined for 10 patients treated with ≥ 110 mg/d navitoclax v best observed clinical response (P = .3 and P = .06, respectively; unpaired two-tailed t test). (D) Shows significant inverse relationship between MCL1 expression in peripheral blood lymphocytes and the maximal reduction in peripheral blood lymphocytosis in the nine patients in whom it could be measured (Spearman coefficient r = −0.63; one-tailed P = .04), as hypothesized. (E and F) MCL1 concentrations and BIM:MCL1 ratios determined for the same patients v best observed clinical response (P = .1 and P =.03, respectively; unpaired two-tailed t test).