Major Shrinking of Residual Tumor Cell Burden and Achievement of Molecular Remissions in Myeloma Patients Undergoing Post-Trasplant Consolidation with Bortezomib, Thalidomide and Dexamethasone: A Qualitative and Quantitative PCR Study

Background and aims. In multiple myeloma (MM) molecular remission (MR) is usually not achieved with autologous transplantation (ASCT) as opposed to allogeneic transplantation where it occurs frequently and associates to an improved outcome. Aim of this study was to assess, by qualitative and quantitative PCR, the impact of a consolidation treatment, including Bortezomib/Thalidomide/Dexamethasone (VTD) on residual MM cells in patients achieving a good clinical response after ASCT.

Patients and methods: Inclusion criteria were:

1. a documented complete or very good partial remission (CR or VGPR) following ASCT delivered as first line treatment;

2. no previous treatment with thalidomide and bortezomib;

3. presence of a molecular marker based on the immunoglobulin heavy chain rearrangement (IgH-R).

VTD had to be started within 6 months from ASCT. Each cycle consisted of:

• Bortezomib 1.6 mg/m2 as an IV injection once weekly (on days 1, 8, 15, 22) followed by a 13-day rest period (days 23–35);

• Thalidomide at the initial dose of 50 mg/day PO once daily, with increments of 50 mg every 7 days up to 200 mg;

• Dexamethasone 20 mg/day PO once daily, on days 1 to 4, 8 to 11 and 15 to 18 followed by a 17-day rest period (days 19–35).

A total of 4 cycles were delivered. MRD was assessed on bone marrow (BM) samples at diagnosis, study entry, after two VTD courses, at the end of treatment and then at six months intervals. Qualitative and quantitative PCR analysis were carried out using IgH-R-derived patient-specific primers as already described (Voena et al, Leukemia 1997; Ladetto et al, Biol Bone Marrow Transpl 2000).

Results: Forty pts were enrolled and are evaluable at study entry. 20% of patients did not receive the whole planned treatment, but were nevertheless included in MRD analysis. Median follow-up from study entry is currently 21 months. Qualitative PCR has been performed on the whole population (overall 198 samples). Six patients converted to MR (defined as two consecutive PCR-negative samples, spaced at least three months) while two patients had an isolated PCR-negative result. Patients in MR persisted in their status with the exception of one molecular relapse at 24 months. No clinical relapse has been so far observed in MR patients at a median follow-up of 26 months (18–30). Among patients not achieving MR we observed eight relapses occurring at a median time of 12 months (4–26). Quantitative PCR has been performed on 20 patients (overall 105 samples). Median tumor bulk at diagnosis was 157000 (35-925000) IgH-R/10⁶ diploid genomes (dg). It shrunk to 440 (3-420000) following ASCT and to 17 (0-113000) following VTD. The effect of VTD was detectable both in patients in VGPR and CR. Follow-up analysis by real time PCR at six, 12 and 18 months in persistently PCR-positive patients revealed stable MRD levels in 66% of patients and a growth greater than one log in 33%. Notably, patients who already relapsed were characterized by a tumor load persistently greater than 100 IgH-R/10⁶dg except one who showed a transient tumor cell reduction after 2 VTD courses followed by a sharp increase in his MRD level.

Conclusions: Post-transplant VTD consolidation is active on residual plasma cells surviving ASCT. Moreover a proportion of CR/VGPR MM enters MR which might persist up to 30 months. Thus, new non-chemotherapeutic agents can substantially improve the quality of remission in MM patients even in case of optimal response to ASCT.