Introduction: Multiple myeloma is an incurable disease caused by the abnormal growth of
plasma cells in the bone marrow and characterised by hypercalcaemia, renal insufficiency,
anaemia, and lytic bone disease. Despite current treatments debulking the majority of tumour
burden, disease relapse often occurs due to minimal residual disease (MRD), including dormant
myeloma cells (DMCs). DMCs can remain inactive for years before being reactivated by the
bone microenvironment (BMME). Previous studies have revealed a crosstalk between DMCs
in the BMME, and several biomarkers have been discovered (e.g., AXL, TRIM44). However,
currently, there is limited knowledge of the effects of myeloma standard of care (SoC) therapies
on DMCs. We hypothesised that DMCs would be targeted more effectively by combined SoC
therapies than monotherapy. To test this hypothesis, both in vitro and in vivo model systems
were used. Methodology: In vitro studies were performed using myeloma cell lines (5TGM1,
JJN3, OPM2, and U266) transduced previously with GFP and/or Luc. Cells were also labelled
with a vybrant membrane dye (DID) to measure and identify DMCs. Flow cytometry and
fluorescent imaging were used to determine optimum time points for drug evaluation on
DMCs. Drug assays were then performed by treating myeloma cells over time with SoC
therapies with different mechanisms of action: Bortezomib (Btz) is a reversible proteosome
inhibitor (PI), Pomalidomide (Pom) is an immunomodulatory (IMiD), Melphalan (Mel) is a
bifunctional alkylating agent, and Panobinostat (Pan) is a histone deacetylase inhibitor
(HDACi). Drug IC50 values were determined and then used to assess the most effective drug
combination to target DMCs in vitro and in vivo. Results: Fluorescent imaging and flow
cytometry showed the presence of DIDhigh cells (potential DMCs) over 21 days in the in vitro
culture of all four myeloma cell lines. JJN3-DIDhigh cells were sensitive to single treatments
and the combination of Btz and Pan compared to single Btz, and OPM2-DIDhigh were sensitive
to single drugs and to the combination of Btz /Mel compared to Btz, Btz/ Pan compared to Btz,
and Mel/ Pan compared to Mel. NSG mice injected with JJN3-GFP-Luc-DID cells and treated
with Btz and/or Pan showed that Btz reduced tumour burden and increased osteoblasts (OB),
thus increasing DMCs. Conclusion: DMCs were present in all myeloma cell cultures,
combined treatment was more effective than monotherapy in vitro. The in vivo showed someiv
combinations reduced the tumour burden but resulted an increase in DMCs. These unexpected
results remain inconclusive and require further investigation.
