Experience of Generic Imatinib As a First Line Therapy for Patients with Chronic Myeloid Leukemia in a Single Reference Institution

Background: The treatment of chronic myeloid leukemia (CML) was revolutionized by the approval of Gleevec (imatinib mesylate) by the FDA in 2001. In low-middle-income countries, due to economic issues related to the high cost of this treatment, scientific governmental entities started to produce and release a generic imatinib in 2013. High quality data about the security profile and efficacy of generic imatinib treatment are still needed. Aims: We herein aimed to evaluate the 6 years follow up of CML patients treated with generic imatinib as first line therapy. Patients and Methods: We evaluated a retrospective cohort of 39 patients diagnosed with CML at a single institution, during the period between December 2001 and July 2019, that had used only generic formulation of imatinib since diagnosis; and analyzed their rate of response to treatment as a primary goal and adverse events and survival outcomes as secondary goals. Responses were evaluated according to ELN 2013. Event-free survival and overall survival were measured from starting date of treatment until: loss of molecular response or death from any cause, and until death from any cause or last seen, respectively. Results: The cohort of 39 patients treated with generic imatinib as first line therapy was composed of 23 men (59%) and 16 women (41%), with median age at diagnosis of 52 years (16-74). The median follow up time was 24 months (8-68), and the median duration of generic imatinib therapy was 19 months (5 - 68). Most of the patients were diagnosed at chronic phase (92%), with only 2 accelerated phase and 1 myeloid blast crisis. Risk stratification according to Eutos, Sokal and Hasford score was low in 92%, 67% and 80%; intermediate in 0%, 30% and 6% and high in 8%, 3% and 14%, respectively. Six different brands of generic imatinib were used (Cristalia, Instituto Vital Brasil, FURP, EMS, Fiocruz and Eurofarma); the most frequently used were Cristalia and Instituto Vital Brasil. The median number of brands used per patient was 2 (1-5). Patients received 400 mg of generic imatinib daily; the dose was increased to 600 mg in 4 patients due to sub-optimal response during follow up. The rate of hematologic response with treatment was 97% and median time to reach it was 1 month (1-7). The rate of response at 3, 6 and 12 months was 74%, 60%, and 92% for optimal cytogenetic response, and 69%, 61%, and 26% for optimal molecular response. The probability to reach deep molecular response at each year of follow up was 41% at 1^st year, 52% at 2^nd year, 46% at 3^rd year, 50% at 4^th year, 50% at 5^th year, and 50% at 6^th year. The probability to reach a molecular response 4.5 at each year of follow up was 10% at 1^st year, 23% at 2^nd year, 30% at 3^rd year, 50% at 4^th year, 50% at 5^th year, and 50% at 6^th year. Hematologic toxicities were frequent during the first three months of therapy. Reported non-hematologic adverse events were hypophosphatemia (62%), diarrhea (30%), cramps (30%), liver toxicity (28%), nausea (18%), bone pain (18%), edema (15%), rash (8%), and hypomagnesaemia (2.5%). Eight percent of patients evolved with deterioration of renal function during the treatment period, but its relationship with generic imatinib was not well established. Two patients (5%) needed a dose reduction because of adverse events. Eight (20.5%) patients switched to second line tyrosine-kinase inhibitors, five (13%) due to resistance and three (8%) due to side effects (severe hepatotoxicity, diarrhea, and rash). Three patients progressed after switching to another tyrosine kinase inhibitor. After a median follow up of 24 months, the event free survival rate was 80% and the overall survival rate was 100%. Conclusion: The rate of complete cytogenetic response, resistance, and intolerance after use of generic imatinib was not worse than the rates described in the long-term follow up of the IRIS trial (N Engl J Med 2017; 376:917-927). Deep molecular response rates seen in the cohort of patients on generic imatinib were inferior to the ones in the IRIS trial, but overall survival was not impacted. Hypophosphatemia was observed in a high percentage of patients, although it has not been reported in other cohorts on generic imatinib. Prospective randomized studies are needed to allow better conclusions regarding the comparative efficacy and safety of generic imatinib.