Homoharringtonine in Combination with Imatinib for Patients with CML Who Have Achieved Partial or Complete Cytogenetic Responses.

Homoharringtonine is a plant alkaloid that inhibits the synthesis of proteins and results in apoptosis. In vitro homoharringtonine shows synergistic or additive effects with imatinib in CML cell lines and in blast cells derived from patients with advanced disease. We undertook a phase I/II study in patients who had achieved a sub-optimal response to imatinib alone to investigate whether the addition of semi-synthetic homoharringtonine (sHHT) (Myelostat®) would reduce the level of residual disease. Patients with CML who had achieved at least 35% Ph-negativity on imatinib were included. All patients were treated with imatinib at ≥ 400mg/day for at least 2 years; and achieved a plateau in BCR-ABL transcripts defined by measuring BCR-ABL transcripts on at least 4 occasions over a minimum period of 1 year with the latest value not lower than the previous minimum value. When sHHT was introduced, imatinib was continued at the previous dosage. sHHT was given subcutaneously at a dose of 1.25 mg/m² twice daily for one day. Courses were repeated every 28 days. The dose of sHHT was escalated by adding one day of treatment every two courses (eg dose level 2: 1.25mg/m² BD x 2 days) if the ANC was ≥2.5x10⁹/l and the platelet count ≥200x10⁹/l by day +28 of the previous course, or adding half a day if the ANC was ≥1.5x10⁹/l and the platelet count ≥ 100x10⁹/l. Patients were considered to have reached the maximal tolerable dose (MTD) when the sHHT could not be escalated further. Response was assessed by serial monitoring of peripheral blood levels of BCR-ABL transcripts assayed by RT-PCR at the start of each course.

Results 10 patients have been enrolled so far. Patients received a median of six courses of sHHT (range 4–8). The MTD has been achieved in all cases (9 cases in account of myelosuppression and in one case in account of grade III asthenia). The median MDT was 2 days (range 1.5–3 days). The transcript level decline in all patients (table 1). In 7 patients the transcript levels declined by >0.5 log and in 5 patients by >1 log with respect the baseline value. In 2 patients transcripts became undetectable. The combination was relatively well tolerated with the principal toxicities being grade II asthenia (n=9), grade III astenia (n=1), grade III neutropenia (n=3) and grade III thrombocytopenia (n=1). We believe these preliminary clinical observations justify further study of the use of sHHT in patients on imatinib who fail to obtain low levels of minimal residual disease.

Table 1