In the modern era for MM treatment, transplant trials strongly support the use of upfront ASCT in the context of novel agents.

The current standard conditioning regimen is high-dose MEL.

The aim of the present study is to evaluate whether a second ASCT with MEL and THIO is safe and feasible in patients with newly diagnosed MM whose first conditioning regimen only used MEL.

The engraftment kinetics and toxicities, seen for THIO/MEL conditioning, are similar to those reported previously with MEL alone.

The association of THIO and MEL does not appear to significantly increase mucositis risk.

TRM was 0% at +100 days post-transplantation.

In the comparison between the first and the second transplant, there was no difference in terms of toxicity and bone marrow engraftment.

Our results suggest that THIO/HDM is a feasible and safe conditioning regimen for ASCT in MM and should be explored for efficacy in a phase III study.