Bosutinib As Therapy for Chronic Phase Chronic Myeloid Leukemia Following Failure with Imatinib Plus Dasatinib and/or Nilotinib: 24-Month Minimum Follow-up Update

Abstract 3785

Bosutinib (BOS) is an orally active, dual Src/Abl tyrosine kinase inhibitor (TKI). This open-label, phase 1/2 study evaluated BOS in patients (pts) with chronic phase chronic myeloid leukemia (CP CML) following TKI failure.

A total of 119 pts aged ≥18 y with prior imatinib (IM) failure plus dasatinib (DAS) resistance (n = 38), DAS intolerance (n = 50), nilotinib (NIL) resistance (n = 27), NIL intolerance (n = 1), or failure of DAS and NIL (n = 3) received BOS starting at 500 mg/d. Median age was 56 y (range, 20–79 y); 45% of pts were male; median time from CML diagnosis was 6.5 y (range, 0.6–18.3 y). Median BOS duration was 8.6 mo (range, 0.2–60.8 mo); 24% of pts are still on treatment. Dose escalation to BOS 600 mg/d occurred in 19% of pts. Time from last pt's first dose to data cutoff was 25 mo (median follow-up duration of 31.4 mo [range, 0.3–66.0 mo]).

A confirmed complete hematologic response (CHR) was attained/maintained by 73% of evaluable pts (Table). The Kaplan-Meier (KM) probability of maintaining a CHR at 2 y was 67%. A major cytogenetic response (MCyR) was attained/maintained by 41%, including 32% with a complete cytogenetic response (CCyR). Among evaluable pts without a baseline CCyR, 36% (n = 37/102) achieved a MCyR, including 28 (28%) with a CCyR. The KM probability of maintaining a MCyR at 2 y was 71%.

Of 86 pts with baseline mutation status, 40 (47%) pts had 19 unique Bcr-Abl kinase domain mutations, including 7 (8%) pts with T315I. Responses were seen across mutations (75% CHR, 43% MCyR excluding T315I), including those conferring resistance to other TKIs; responses in pts with T315I were low (29% CHR; 14% MCyR).

Nine of 37 pts evaluated at baseline and treatment discontinuation had ≥1 new mutation (V299L, n = 4; L248V, n = 2; T315I, n = 2; F359C, n = 1; G250E, n = 1); 8 of 9 pts had discontinued BOS due to disease progression or lack of efficacy.

On-treatment transformation to accelerated phase CML occurred in 5 (4%) pts after 16 to 428 d on study; no pt transformed to blast phase CML. KM-estimated on-treatment progression-free survival (PFS) at 2 y was 75%; KM-estimated overall survival (OS) at 2 y was 84% (Table). There were 23 (19%) deaths on study, with 6 deaths occurring ≤30 d after the last BOS dose. Most deaths were due to disease progression (n = 10 [8%]) or an adverse event (AE; n = 10 [8%]; including 1 treatment-related death due to gastrointestinal bleeding). Three deaths were due to unknown cause ≥509 d after the last BOS dose.

Non-hematologic treatment-emergent AEs (TEAEs) seen in ≥20% of pts (all grades; grade 3/4) included diarrhea (82%; 8%), nausea (49%; 1%), vomiting (40%; 1%), rash (27%; 3%), headache (26%; 3%), fatigue (24%; 1%), and abdominal pain (20%; 1%). The incidence of individual TEAEs was generally similar across groups regardless of prior TKI exposure. Diarrhea TEAEs were predominantly grade 1/2, first reported early during treatment (median time to first event of 1.5 d [range, 1–210 d]), and transient (median event duration of 2 d [range, 1–524 d]). The incidence of pleural effusion was highest among DAS-intolerant pts (n = 11 [22%], including 3 pts with grade 3 events); for 9 of 11 pts pleural effusion had been indicated as a reason for intolerance to prior DAS. Grade 3/4 laboratory abnormalities reported in ≥10% of pts included thrombocytopenia (25%), neutropenia (19%), lymphopenia (17%), and hypermagnesemia (12%). Dose reductions and interruptions were used to manage AEs in 50% and 66% of pts. A total of 32 (27%) pts discontinued treatment due to an AE, most commonly hematologic events.

In conclusion, BOS therapy continues to demonstrate durable efficacy and manageable toxicity after follow-up of ≥24 mo in CP CML following resistance or intolerance to multiple TKIs, with a majority of pts maintaining response at 2 y and few new transformations, deaths, TEAEs, or discontinuations due to AEs since the prior report ∼1 y earlier (Blood 2012;119:4303–12).