Translational Research of PT-112, a Clinical Agent in Advanced Phase I Development: Evident Bone Tropism, Synergy In Vitro with Bortezomib and Lenalidomide , and Potent Efficacy in the Vk*MYC Mouse Model of Multiple Myeloma

Introduction: PT-112 is a first-in-class platinum-pyrophosphate agent under advanced Phase I development in solid tumors. PT-112 was rationally designed to circumvent the toxicity and mechanisms of resistance associated with conventional chemotherapy. In previous preclinical models using PT-112, multiple cell signaling effects have been observed: p16 mediated G1/S cell cycle arrest; MDM2/p53 expression modulation; extrinsic apoptosis initiation; and signals indicative of immunogenic cell death (Ames et al., Eur. J. Cancer 2016). In addition, the ongoing first-in-human PT-112-101 Phase I trial in solid tumors has demonstrated a positive therapeutic index, with a range of biological activity observed at well-tolerated dose levels and a lack of severe hematotoxicity (Karp et al., J. Clin. Oncol. 2017).

Due to the pyrophosphate present in PT-112, we hypothesized that PT-112 should localize to bone tissue, and be efficacious against bone marrow malignancies such as multiple myeloma (MM). Biodistribution was measured via laser ablation inductively-coupled plasma mass spectrometry (LA-ICP-MS), along with translational experimentation in the orthotopic Vk*MYC mouse model of MM. In addition, an assessment of synergy was conducted in vitro in combination with standard of care (SoC) agents such as bortezomib (BOR) and lenalidomide (LEN).

Methods: In vitro - Cell viability and IC₅₀ values in MM1R and RPMI-8226 suspension cells were measured after treatment with PT-112, LEN, BOR, or PT-112-containing combinations thereof. Combination indices to assess synergy were calculated using the Chou-Talalay method.

Biodistribution - After a single dose with 90 mg/kg PT-112 or vehicle control via tail vein injection, mice were euthanized 45min or 24h post-dose and snap frozen. Slides were prepared and LA-ICP-MS was used to measure and image the presence and concentration of platinum (Pt) in full sagittal plane cross sections, and in individual organs and tissues at higher spatial resolution.

In vivo - Genetically engineered de novo Vk*MYC mice with established MM were treated with either 100 mg/kg PT-112 twice weekly (n=2) or 67 mg/kg (n=1) thrice weekly via intraperitoneal injection. In parallel experiments, mice engrafted with the BOR resistant Vk12598 transplantable line received vehicle (n=11) or PT-112 at 62.5 mg/kg (n=12) twice/week. M-spike levels were monitored weekly during treatment and for several weeks after.

Results: In vitro, the combination of PT-112 with LEN and BOR resulted in strong synergy in both RPMI-8226 and dexamethasone-resistant MM1R multiple myeloma cells, as indicated by combination indices as low as 0.0029. In vivo, PT-112-derived Pt was observed in kidney, lung, liver and other tissue types, but most prominently in bone. All bone tissue observed had high concentrations of Pt, with the highest amount reaching ~15mM, a concentration >10³ fold higher than the IC₅₀ of PT-112 in in vitro cell viability experiments.

PT-112 treatment induced a response in all of the de novo Vk*MYC mice with a >50% reductions in M-spike at the 2-week time point, passing the statistically correlated activity threshold indicative of robust activity in human MM patients (Chesi et al., Blood 2012). At this point treatments were discontinued, and M-spikes levels continued to decline for 1-2 weeks, with the lowest level observed at 19% of baseline at 11 days after treatment discontinuation. Importantly, we also noted pronounced single agent PT-112 activity against the BOR refractory Vk12598 tumors, resulting in statistically significant improved overall survival. In vivo combination studies are ongoing.

Conclusions: Sophisticated biodistribution imaging techniques reveal the presence of PT-112 derived Pt in several target tissues, with a demonstrably preferential accumulation in bone tissue. These biodistribution results likely explain in part the strong efficacy observed in Vk*MYC mice. Given the predictive power of this model, the likelihood of PT-112 activity in human MM patients is high. Additionally, the potent synergy observed in in vitro systems provides encouraging evidence that PT-112 might combine well with SoC agents in treatment of MM. Taken together, these data provide a strong rationale for study of PT-112 as both a single agent and in combination in MM patients, with a further prospect for investigating other hematological malignancies.