Subcutaneous PAD As Induction Therapy for Patients with Newly Diagnosed Myeloma: A Phase 2 Trial Assessing the Impact of Minimal Residual Disease (MRD) in Patients with Deferred Autologous Stem Cell Transplantation (PADIMAC)

Introduction: The role of autologous stem cell transplantation (ASCT) as first line treatment for newly diagnosed patients with myeloma is currently under evaluation given the high response rates to novel induction treatment. The outcomes for patients that do not proceed to ASCT following induction remain unclear and are likely to be determined by genetic risk and response to therapy. In order to evaluate this further, this single arm phase 2 clinical trial conducted at 13 sites in the UK was designed to determine the 2 year progression free survival for patients that achieved ≥ very good partial response (VGPR) following induction therapy without ASCT. Those achieving partial response (PR) were consolidated with ASCT according to routine practice. In this first analysis we report secondary endpoints: disease response and regimen-related toxicity in patients completing induction, including minimal residual disease (MRD) negativity by multiparameter flow cytometry.

Methods: Patients with newly diagnosed myeloma eligible for ASCT received PAD (bortezomib 1.3mg/m² days 1, 4, 8, 11; doxorubicin 9mg/m² days 1-4 and dexamethasone 40mg days 1-4 (and days 8-11 and 15-18 for the first cycle only)) for up to 6 cycles (minimum of 4). Bortezomib was initially given intravenously (IV), but once approved this was switched to sub-cutaneous (SC). Those failing to achieve PR were offered salvage therapy off protocol. All others had peripheral blood stem cell (PBSC) mobilisation using cyclophosphamide + GCSF, followed by MRD assessment on bone marrow aspirates using multi-parameter flow cytometry. Depending on disease response, patients were then stratified to ASCT (PR) or no further treatment (≥VGPR). Responses were assessed using International uniform response criteria (Durie 2006) by intent-to-treat and toxicity scored according to CTCAE version 4.0. High risk disease was defined by the presence of one or more adverse FISH lesions (t(4;14), t(14;16), t(14;20), del(17p13), +1q21) as described in the MRC Myeloma IX trial.

Results: Between March 2011 and January 2014, 153 patients were enrolled (median age of 55, range 28-71 years). 139 (91%) received between 4-6 cycles of PAD. The majority (135, 88%) received SC only bortezomib and 18 (12%) received at least 1 cycle IV. The overall response rate to PAD was 81% with 46% achieving ≥VGPR (sCR: 6 (4%), CR: 13 (8%), VGPR: 51 (33%), PR: 54 (35%)). FISH data was available for 122 patients, 91 (75%) patients were standard risk and 31 (25%) were adverse. Responses were similar irrespective of ISS or genetic risk (standard, ≥VGPR 44%, PR 34%; adverse, ≥VGPR 55%, PR 29%). MRD results are currently available in 70 of the 124 patients achieving PR post PBSC harvest. Of this group 41 achieved ≥VGPR post-harvest (22 MRD+ and 19 MRD-) and hence did not proceed to ASCT, 13 patients achieved CR of which 8 were MRD negative. Toxicity was as expected for PAD and predominantly haematological. Notably the incidence of neuropathy was lower than that previously reported with IV bortezomib. Grade 3-4 events were: neutropenia: 18%; thrombocytopenia 7%. Grade 2-4 peripheral neuropathy was reported in 27% compared to 40% in the HOVON-65/ GMMG-HD4 Trial using IV bortezomib. Grade 1-2 neuropathy was similar for patients who received IV (55.6%) or SC (60%) bortezomib; however only 7% of patients receiving SC bortezomib developed grade 3 neuropathy compared to 28% with the IV route.

Conclusions: SC PAD is a highly effective induction regimen for patients with newly diagnosed myeloma achieving a ≥VGPR of 46%. Of the 41 patients achieving ≥VGPR post-harvest with MRD result available, 46% were MRD negative. Response rates were similar across ISS and with adverse FISH. The use of SC bortezomib improved tolerability and substantially reduced neurotoxicity. ISRCTN no: 03381785.