Carfilzomib Platform Study Using The LAGκ-1A Multiple Myeloma Xenograft Model
Authors: Sanchez Eric , Li Mingjie , Vardanyan Suzie , Gottlieb Jillian , Delijani Kevin , Ben-Zvi Joseph , Song Mitchell , Halleluyan Ran , Wang Cathy S , Chen Haiming , Berenson James R. 
Published: October 2019

Introduction
We previously demonstrated that severe combined immunodeficient (SCID) mice bearing the human multiple myeloma (MM) xenograft LAGκ-1A treated with single agent carfilzomib or the alkylating agent (AA) cyclophosphamide (CY) did not show a reduction in tumor growth compared to vehicle-treated mice. In contrast, carfilzomib with CY resulted in a significant decrease in tumor size and IgG levels when compared to mice treated with single agent carfilzomib or CY or vehicle alone. We have also shown that the combination of carfilzomib and another AA, bendamustine, decreased tumor size and IgG levels, when compared to mice treated with single agents or vehicle alone. However, no data is available regarding sequencing of the proteasome inhibitors (PI) carfilzomib or bortezomib with the AA melphalan (MEL). Thus, we used our SCID-hu MM models to evaluate the sequencing of these drugs with MEL. These studies are critical as both PIs are now being used to treat MM. Thus, we evaluated the response, toxicity and survival of animals treated sequentially with these drugs.

Methods
Each naïve SCID mouse was surgically implanted with a 20 – 40 mm3 MM tumor piece into the left hind limb superficial gluteal muscle. Seven days post–implantation mice were randomized into treatment groups based on human immunoglobulin (Ig) G levels. Carfilzomib stock solution (2 mg/ml) was diluted to 3 mg/kg using 5% dextrose and administered twice weekly on two consecutive days via intravenous (i.v.) injection. Bortezomib stock solution (1 mg/ml) was diluted to 0.25 mg/kg using NaCl and administered twice weekly (Thursdays and Saturdays) via i.v. injection. MEL stock solution (3 mg/ml) was diluted to 1 mg/kg using PBS and administered once weekly via intraperitoneal injection. Mice (n = 10/group) were initially treated with carfilzomib or MEL alone until tumor progression. Progression was defined as an increase in paraprotein equal to or above 25% confirmed on one consecutive assessment. Mice initially treated with carfilzomib were randomized to continue to receive single agent carfilzomib, add in MEL alone or combine it with ongoing carfilzomib, substitute single agent bortezomib, or discontinue treatment altogether. A similar treatment strategy was evaluated with mice treated initially with MEL. At progression, these animals were continued on single agent MEL, carfilzomib added alone or with continuation of MEL, or discontinued treatment. Tumor size was measured using standard calipers and human IgG levels with an ELISA (Bethyl Laboratories, Montgomery, TX). This study was conducted according to protocols approved by the Institutional Animal Care and Use Committee.

Results
When carfilzomib was administered first, followed by the addition of MEL, a modest nonsignificant reduction in tumor size was observed compared to either drug alone. In addition, substitution of single agent bortezomib for carfilzomib showed no effect on tumor size. However, when MEL was administered first and carfilzomib was added after disease progression, at days 35 and 42 (end of study) post tumor implantation, mice treated with the combination showed a reduction in tumor volume compared to mice that discontinued melphalan (P = 0.0378 and P = 0.0105, respectively) whereas mice treated with carfilzomib alone showed no reduction in tumor size following progression from MEL. Notably, throughout the study there was a trend toward smaller tumors in mice receiving this combination when compared to mice receiving single agent treatment with carfilzomib or MEL alone or vehicle. Similar effects were observed on human IgG levels. Overall, all mice survived combination or single agent treatment with these agents.

Conclusions
These in vivo studies using our human MM LAGκ–1A SCID–hu model show that animals progressing from initial MEL treatment show a reduction in MM tumor burden when carfilzomib is added to MEL at progression. In contrast, mice progressing from initial carfilzomib treatment did not benefit from the addition of MEL at disease progression. No drug-related deaths occurred in any treatment group. This study demonstrates that using a different MM model (LAGκ-1A), that the PI carfilzomib can produce anti-tumor effects among mice progressing from single-agent MEL treatment, providing further support for the use of this PI as an agent that can help overcome drug resistance in MM.

Disclosures:
Berenson: Onyx Pharmaceuticals: Consultancy, Honoraria, Research Funding, Speakers Bureau.