A Phase I/II Study of Bortezomib and Low Dose Intravenous Melphalan (BM) for Relapsed Multiple Myeloma.

Background: Bortezomib as a single agent has known efficacy in the treatment of relapsed multiple myeloma. The overall response rate (CR+PR+MR) was 35% in the SUMMIT study and 46% in the APEX study. In-vitro studies including our own have demonstrated potent synergy with other chemotherapeutic agents such as melphalan. It therefore follows that responses to bortezomib may be further improved by the combination of such drugs.

Aims: The primary objectives of this Phase I/II study was to assess the safety, tolerability and response rates in patients with relapsed multiple myeloma; secondary objectives being time to progression (TTP) and overall surival (OS).

Methods: This was a multi-centre, non-randomised trial for patients with relapsed myeloma. Patients received bortezomib 1.3mg/m² on days 1,4,8 and 11 of each 28 day cycle with melphalan on day 2 at increasing dose levels. This was initially at 10mg/m², but due to cytopenias subsequently at 2.5 and 5mg/m² (levels 1a, 1 and 2) and we plan to escalate to 7.5mg/m². Up to 8 cycles were given with dexamethasone added for stable or progressive disease after 4 or 2 cycles respectively. Responses were determined by EBMT criteria.

Results: To date, 18 patients have been enrolled (12 male 6 female; median age 60 [range 44–73]; median number of prior therapies 3 [range 1–5] of which 17 have had at least one autologous stem cell procedure with high dose melphalan; 10 prior thalidomide and 2 prior bortezomib). 12 patients received melphalan at 10mg/m² but due to unacceptable delays predominantly due to thrombocytopaenia, subsequent treatment levels commenced at 2.5mg/m². The median number of cycles completed thus far is 4 (range 0–8) and of the 16 evaluable, the overall response rate (CR+PR+MR) across all treatment levels was 50% rising to 75% following the addition of dexamethasone as per protocol. At level 1a (melphalan 10mg/m² ,N=12, median number of cycles completed =5) the best responses (with dexamethasone as indicated) were: 1CR, 1 VGPR, 5 PR, 2 MR; at level 1 (melphalan 2.5mg/m^2, N=4) 1 PR, 2 MR (after 2 cycles only). The median time to any response was 1 cycle (range 1–3 ). Three patients have progressive disease, but the median TTP and OS have not yet been reached (median follow-up 3 months).

Non-haematological toxicities have been modest with 7 SAEs reported of which only 1 was possibly drug related (myocardial infarction), and 4 episodes of Grade 3 neuropathy (2 resulting in study withdrawal). The commonest grade 3–4 haematological toxicity was thrombocytopaenia (N=10) complicated by bleeding in one patient, followed by neutropenia (N=6).

Summary: The combination of bortezomib and intravenous melphalan can be given safely to patients with relapsed multiple myeloma and dose escalation is ongoing. Myelosupression was the commonest grade 3–4 adverse event. A response rate of 50% was seen, which was further improved to 75% with the addition of dexamethasone. This combination may therefore result in higher responses than single agent bortezomib in heavily pretreated patients.