Glasdegib in Addition to Intensive or Non-Intensive Chemotherapy in Patients with Acute Myeloid Leukemia: Safety Analysis of Glasdegib 'On Target' Adverse Events

Background: Novel agents are frequently added to standard acute myeloid leukemia (AML) treatment backbones yet it is unclear how much additional toxicity this introduces, with the potential for adverse events (AEs) to be caused by the backbone chemotherapy or the disease itself. Glasdegib (PF-04449913) is an investigational, oral small molecule inhibitor of the Hedgehog (Hh) pathway component Smoothened (SMO), currently in clinical development for AML treatment. In a Phase 2 randomized study, addition of glasdegib to low-dose cytarabine (LDAC) improved overall survival (OS) vs LDAC alone and combination therapy was generally well tolerated with minor differences in AE rates vs chemotherapy alone. Here, we analyzed specific 'on target' AEs consistent with inhibition of the SMO component of the Hh pathway to assess impact on overall toxicity.

Methods: We pooled safety data from both portions of a Phase 1b + Phase 2 multicenter study (NCT01546038), including AML patients assigned to glasdegib 100 mg QD with LDAC (non-intensive treatment) or with cytarabine/daunorubicin on a 7+3 schedule (intensive treatment) (figure 1). We assessed 'on target' all-causality treatment-emergent AEs of muscle spasms, alopecia, and dysgeusia. We also compared AE onset in defined study time-periods (non-intensive: 0-6, 6-12, or >12 months; intensive: induction, consolidation, maintenance); defining long-term treatment as >12 months for non-intensive and maintenance for intensive.

Results: Across studies, 93 patients were enrolled in the non-intensive group and 80 in the intensive group. Here, we focused on patients treated with glasdegib: 89 patients in the non-intensive group and 78 in the intensive group. Table 1 shows baseline characteristics; median treatment duration was 69 days (range 3-1280) and 51 days (10-539), respectively. Rates of treatment discontinuation due to all AEs (inclusive of 'on target' and others), deemed by the investigator to be related to study drug (glasdegib and/or backbone chemotherapy) were similar (non-intensive, 10 patients [11.2%]; intensive, 15 patients [19.2%]). Frequency of muscle spasms was similar for the 2 groups; observed in 19 of the non-intensive group (21.3%) and 18 (23.1%) of the intensive group (table 1), with few grade 3 events (non-intensive 4.5%; intensive 1.3%), and the number of patients who developed muscle spasms (nearly all grade 1) when exposed to long-term treatment was small (4 non-intensive patients; 7 intensive patients) with no cases of rhabdomyolysis. Alopecia was reported for 8 (9.0%) of the non-intensive group and 16 (20.5%) of the intensive group (table 1). Alopecia had earlier time to onset in the intensive arm than in the non-intensive arm (table 1), likely reflecting the concomitant chemotherapy. Alopecia was less frequent during glasdegib maintenance monotherapy (5.3%) in the intensive arm than when given with 7+3 (16.7%). Dysgeusia was similar for the 2 groups, observed in 22 of the non-intensive group (24.7%) and 24 (30.8%) of the intensive group (table 1). For both groups, dysgeusia had similar time to onset and was less common during long-term treatment. The number of patients having a dose modification as a consequence of class-related AEs was low (for non-intensive and intensive, respectively: dose reduction, 5.6% and 2.6%; temporary discontinuation, 4.5% and 2.6%; permanent discontinuation, 1.1% and 2.6%).

Conclusions: Glasdegib 'on target' AEs of muscle spasms, alopecia, and dysgeusia were mainly of mild severity, had infrequent dose modifications or permanent discontinuations, and did not appear to impair tolerability of combination treatment. In comparison, muscle spasms and dysgeusia occurred in ≤5% for the LDAC alone arm, and no alopecia was reported. Although muscle spasms and dysgeusia occurred with similar frequency, dysgeusia occurred earlier, for a shorter duration, and was more persistent at the time of discontinuation compared with muscle spasms. Similar safety profiles were observed when combining glasdegib with LDAC or 7+3, suggesting that glasdegib in combination with standard chemotherapy has a manageable safety profile and thus can be an acceptable combination partner in the treatment of AML. Our results are consistent with previously reported safety outcomes for glasdegib as monotherapy in hematologic malignancies. Clinical trials of glasdegib in combination with other standard of care AML agents are ongoing.

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