Inecalcitol Stimulates CD38 Expression in Multiple Myeloma and Acute Myeloid Leukemia Cell Lines

Inecalcitol (Hybrigenics) is a vitamin D receptor agonist already characterized by a high anti-proliferative effect and a low calcemic potential (1), allowing its administration at high oral doses to human cancer patients (2), currently in Phase II clinical trial in AML. 

The present findings show upregulation of the CD38 protein on the surface of multiple myeloma (MM) cells and acute myeloid leukemia (AML) cells. 

CD38 is the target of daratumumab, the first therapeutic anti-CD38 monoclonal antibody approved in 2015 for relapsed refractory MM (Darzalex®, Genmab and Janssen) (3). Interestingly, the pretreatment levels of CD38 expression on MM cells were significantly higher in patients who achieved at least partial response (PR) as compared to patients who achieved less than PR (4). Therefore, increasing the levels of CD38 expression is the next logical pharmacological step to enhance the efficacy of anti-CD38 therapy. 

The human MM cell line MM1.S showed a mix of CD38 positive (CD38+) and CD38 negative (CD38-) cells, as illustrated by the distribution spectrum of intensities of individual cell labeling. After treatment by inecalcitol, all MM.1S cells became CD38+ and the total concentration of CD38 on their surface was multiplied by 5 (quantitative cytometry: 200,000 CD38 antibodies bound per cell after 72h treatment, versus 40,000 on vehicle treated cells). The EC50 of CD38 increase triggered by 72h inecalcitol treatment was 0.5 nM.

It was also found that, despite being mostly CD38-, all cells from the human AML cell line HL-60, became CD38+ after treatment by inecalcitol and the intensity of their CD38 labeling was multiplied by 10 (quantitative cytometry: 30,000 CD38 antibodies bound per cell after 72h treatment, versus 3,000 on vehicle treated cells). The EC50 of CD38 increase triggered by 72h inecalcitol treatment was 0.3 nM.

The time-course of the activity of inecalcitol has been investigated: its maximal effect is observed as early as 48 hours after the start of treatment of the AML HL-60 cell line and 72 hours with the MM MM.1S cell line. 

Inecalcitol upregulates CD38 mRNA (qPCR, results not shown), leading to increased protein amount (Western Blot).

Since these initial studies on 2 cell lines, similar results have additionally been reproduced on 4 more MM cell lines (MM.1R, H929, L-363 and RPMI-8226) and 3 more AML cell lines (U-937, MOLM-13 and THP1). Only the U266 MM cell line remained insensitive to inecalcitol. CD38 was not upregulated in other hematologic and non-hematologic cell lines.

Taken altogether, these results demonstrate an early, potent and reproducible induction of CD38 by inecalcitol on the surface of MM and AML cell lines. 

These findings strongly support the idea to test in the clinics the combination of inecalcitol with an anti-CD38 therapy such as daratumumab. Inecalcitol may strengthen the use of daratumumab in multiple myeloma and could also open acute myeloid leukemia as a new therapeutic indication for daratumumab.

1-Okamoto R et al., 2012, Int J Cancer; Ma Y et al., 2013, Cell Cycle

2-Medioni J et al, 2014, Clin Cancer Res

3-Sanchez L et al., 2016, J Hematol Oncol; van de Donk NWC et al., 2016, Immunol Rev

4-Nijhof IS et al., 2016, Blood