The Combination of Cyclophosphamide and Thalidomide During Induction Therapy for Multiple Myeloma Results in a High Rate of Stem Cell Mobilization Failure.

Abstract 2147

Poster Board II-124

In the era of novel therapeutic agents, high-dose chemotherapy and autologous stem cell transplantation (ASCT) remains an integral component of treatment for multiple myeloma (MM), with a proportion of patients undergoing more than one ASCT during the course of their disease. Therefore, the choice of new drug combinations for induction therapies must take into consideration the requirement to collect a sufficient number of stem cells, which is also reflected in a recently published consensus perspective of the International Myeloma Working Group. The immunomodulatory drug Lenalidomide and the alkylating agent Melphalan have a substantial impact on stem cell mobilization, but the effect of induction therapies containing either Thalidomide or Cyclophosphamide on the stem cell collection yield is negligible. We considered the possibility that the combination of Cyclophosphamide and Thalidomide, which is widely used as an induction regime particularly in the UK as part of the CTD regime (with Dexamethasone), could have an additive impact on the stem cell pool and cause mobilization failures. We carried out a retrospective analysis of the outcome of peripheral blood stem cell mobilizations in MM patients performed at our institution over a four-year period in patients who had received CTD (n=55), and compared them with a control group of patients (n=56) who had received VAD (Vincristine, Doxorubicin, Dexamethasone; n=30) or Z-Dex (Idarubicin, Dexamethasone; n=26) during the same period. There were no differences between the CTD and control group in terms of age, MM subtype, disease stage, or remission status at the time of stem cell mobilization. All mobilizations were performed with Cyclophosphamide (4g/m²) and G-CSF (5-10μg/kg). Apheresis was attempted when the peripheral blood CD34 count was >10 × 10³/ml, and the standard harvest target was 4 × 10⁶ CD34+ cells/kg, with a minimal target of 2 × 10⁶ CD34+ cells/kg. The total number of CD34+ cells harvested was substantially lower in the CTD group (5.2 vs. 9.7 × 10⁶/kg, p=0.002), and a higher number of patients in the CTD group underwent more than one apheresis procedure (52.8% vs. 32.1%, p=0.012). The number of CD34+ cells harvested on the first day of apheresis and per apheresis procedure were also lower in the CTD group (2.8 vs. 7.3 × 10⁶/kg, p=0.002; 2.6 vs. 6.7 × 10⁶/kg, p=0.002). More patients in the CTD group failed to achieve both the standard (36.4% vs. 16.1%, p=0.021) and minimal (19.2% vs. 5.4%, p=0.036) stem cell harvest target. The failure rate on the first day of apheresis was also higher in the CTD group both for the standard (56.3% vs. 28.6%, p=0.003) and the minimal target (36.7% vs. 16.1%, p=0.041). There was no difference in stem cell yield between the VAD and Z-Dex groups. Age and number of induction chemotherapy cycles did not have an impact on mobilization failure in the entire cohort or the CTD group alone. In the CTD group, 18% of patients underwent re-mobilization with Etoposide (1.6g/ m²) and G-CSF (n=8), or with Plerixafor (240μg/kg) and G-CSF (n=2), which was successful in all patients. These results demonstrate that the CTD induction regime results in a high rate of stem cell mobilization failures, which is associated with the requirement for an increased number of apheresis and re-mobilization procedures. The observations provide novel evidence that drugs with no previously demonstrated significant effect on stem cell mobilization can have a considerable negative impact on the stem cell yield when used in combination. The possible benefit of new drug combinations has to be balanced against the increase in cost, the potentially higher rate of complications, and delays or failures to progress to ASCT resulting from impaired stem cell mobilization.