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ated significant interest as a therapeutic target in MM. JAK inhibition has already been validated in a number of preclinical studies as a way to target this pathway.14-19 However, the studied compounds are not yet available clinically, and may never be. Herein, following the results of a large-scale repurposing screen, we validated tofaci- tinib as a potential therapy that can be rapidly translated into MM patients.
Using a combination of mechanistic pharmacology and unbiased mass spectrometry-based phosphoproteomics, we found that tofacitinib appears to reverse stromal- induced proliferation of MM plasma cells by inhibiting JAK/STAT signaling. Our results also support the use of unbiased phosphoproteomics both in kinase inhibitor evaluation and more broadly in MM biology. While oth- ers have shown that IL-6 can lead to increased signaling through STAT3,40 our results suggest that additional fac- tors derived from BMSC can lead to plasma cell prolifera- tion. While we were unable to identify these factors, we did rule out a number of cytokines highly secreted by HS5 cells.30 Furthermore, given that murine IL-6 does not cross- react with the human IL-6 receptor,41 our in vivo results also suggest that other factors in the murine marrow microen- vironment can stimulate JAK-STAT signaling, which is then reversed by tofacitinib. Candidates include leukemia inhibitory factor (LIF), which cross-reacts between mouse and humans,41,42 or the complex between murine IL-6 and soluble murine IL-6 receptor, both of which can stimulate JAK/STAT signaling in human cells.43
Surprisingly, in our studies, we found that the FDA- approved JAK1/2 inhibitor ruxolitinib did not lead to the same anti-MM effects as tofacitinib. We do note that rux- olitinib was previously evaluated in a small trial of 13 MM patients in combination with dexamethasone (clinicaltri- als.gov Identifier: 00639002), and no significant anti-MM effects were noted in this small study. Our in vitro results presented herein may provide a partial explanation for the lack of ruxolitinib efficacy in that trial. Another recent study also found minimal anti-MM cell line effects of rux- olitinib unless used at extremely high doses and exposure times.44 However, it remains mechanistically unclear why ruxolitinib is unable to block JAK-STAT signaling in these myeloma models.
We note that our studies are undoubtedly limited in that
they are performed in MM cell lines. While we primarily focused our analysis on stromal-responsive lines, which appear to better mimic the malignant plasma cell pheno- type found in MM patients, focused clinical trials in MM will be necessary at this point to truly evaluate whether tofacitinib has anti-MM effects.
Toward this goal, the value of drug repurposing becomes readily apparent. Tofacitinib can be quickly moved into Phase I/II studies in MM as the tolerated doses and adverse event profiles of this drug are well-character- ized.11 Intriguingly, a patient population with both MM and RA could readily serve as the basis of a multi-center trial. Alternatively, a patient population with early-stage disease, perhaps smoldering myeloma, may be the opti- mal setting for clinical use, when plasma cells may be most dependent on microenvironmental cues. Our find- ings of strong synergy between tofacitinib and venetoclax in the context of the BM microenvironment may be rele- vant given the exciting progress of venetoclax in the clinic.37 This finding may also reflect a general vulnerabil- ity specific to the BM niche, given the effectiveness of this combination both in MM and AML.30 In conclusion, tofac- itinib is a promising agent to reverse the tumor-prolifera- tive effects of the BM microenvironment that can be rap- idly repurposed to benefit MM patients.
Funding
This work was supported by the UCSF Stephen and Nancy Grand Multiple Myeloma Translational Initiative and the Myeloma Research Fund of the Silicon Valley Community Foundation (to BTA and APW), an NCI Cancer Center Support Grant (P30 CA082103) (to BCH), and an NCI Clinical Scientist Development Award (K08 CA184116), a Dale F. Frey Breakthrough Award from the Damon Runyon Cancer Research Foundation (DFS 14-15), and an American Cancer Society Individual Research Award (IRG-97-150-13) (to APW).
Acknowledgments
We thank Drs. Jeffrey Wolf, Tom Martin, Nina Shah, and Cammie Edwards for discussions, advice, and insight. We thank the staff of the UCSF Helen Diller Family Cancer Center Preclinical Therapeutic Core facility for completion of murine studies. We thank Dr. Diego Acosta-Alvear for providing luciferase-labeled MM cell lines.
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