Abstract LB-306: Impacting Humoral Hypercalcemia of Malignancy (HHM) and associated PTH1R-mediated morbidities: Characterization of an anti-PTH1R antagonist monoclonal antibody to reverse hypercalcemia
Authors: Mirza Amer M. , Choppin Agnes , Bedinger Daniel , Hunt Rachel , Cotter Robyn , Pongo Elizabeth , Ahluwalia Kiran , Tran Catarina , Lao Llewelyn , Lind Kristin , Wijesuriya Sujeewa , Webster Lynn , Zhang Fangjiu , Johnson Kirk , Takeuchi Toshihiko , Levy Raphael 
Published: July 2017

Abstract
Humoral Hypercalcemia of Malignancy (HHM) occurs in 20-30 percent of advanced cancers, both solid tumors and hematologic malignancies, and is the most common life-threatening complication of cancer. HHM is due to tumors secreting abnormally high quantities of parathyroid hormone-related protein (PTHrP), a ligand for PTH1R (a family B GPCR) resulting in hypercalcemia. A potent and long acting PTH1R antagonist could reverse the hypercalcemia in HHM. Using XOMA’s fully-human antibody phage display libraries, highly potent anti-PTH1R antagonist monoclonal antibodies were discovered, and screened by FACS for binding to CHO-K1 cells over-expressing human PTH1R. Human and murine receptor antagonism was demonstrated in cAMP accumulation and calcium flux assays with the human Saos-2 and the rat UMR106 osteosarcoma cell lines, which express endogenous PTH1R. The epitope demonstrating the greatest receptor antagonism was localized to the N-terminal extracellular domain of human PTH1R. Antibodies were affinity matured via light chain shuffling, resulting in leads with sub-nanomolar binding affinities as measured by SPR and verified by FACS. Functionally, PTH1R is expressed on osteoblasts and osteocytes. Stimulation with PTHrP leads to increased expression of M-CSF, RANKL and other factors that drive the differentiation and activation of bone resorbing osteoclast cells. It was shown that the lead anti-PTH1R mAb inhibited both PTH- and PTHrP-induced osteoclast differentiation by greater than 10-fold. In vivo proof-of-concept was achieved in rodent models where hypercalcemia was established in rats by SC infusion of PTHrP via an osmotic pump. IV administration of 2 and 10 mg/kg antibody reduced serum calcium levels by a minimum of 2 mg/dL within 48 hours in a dose-dependent manner. Additionally, in a murine C26 tumor model of hypercalcemia associated with elevated PTHrP, the anti-PTH1R mAb showed a dose-dependent correction of hypercalcemia at doses as low as 2 mg/kg with a sustained duration of action. In this model, the antibody given at 10 mg/kg IV completely reversed the hypercalcemia within 24 hours, lowering serum calcium from greater than 15mg/dL to approximately 6mg/dL. Additional PK/PD parameters were assessed in mouse and rat models. Patients with HHM typically have a poor prognosis and HHM is a frequent reason that cancer patients are readmitted to hospitals or enter hospice care. Left untreated, hypercalcemia in these patients can cause coma or death. The highly potent anti-PTH1R antagonist mAb described here has the potential to become a first in class therapy for HHM, ameliorating hypercalcemia-associated morbidities, and extending the utility of other anti-cancer agents. This promising therapeutic antibody is currently in late stage preclinical development.
Citation Format: Amer M. Mirza, Agnes Choppin, Daniel Bedinger, Rachel Hunt, Robyn Cotter, Elizabeth Pongo, Kiran Ahluwalia, Catarina Tran, Llewelyn Lao, Kristin Lind, Sujeewa Wijesuriya, Lynn Webster, Fangjiu Zhang, Kirk Johnson, Toshihiko Takeuchi, Raphael Levy. Impacting Humoral Hypercalcemia of Malignancy (HHM) and associated PTH1R-mediated morbidities: Characterization of an anti-PTH1R antagonist monoclonal antibody to reverse hypercalcemia [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr LB-306. doi:10.1158/1538-7445.AM2017-LB-306