First-in-human experience using RBS2418, an oral ENPP1 inhibitor within an expanded access protocol in combination with pembrolizumab in a patient with metastatic adrenal cancer.
Authors: Csiki Ildiko , Dong Aiwen , Tuan Bertrand Y. , John Elizabeth , O'Toole Leslie , Seppa Judy , Huang Ningwu , Hawley Ronald C. , Exon Chris , Glenn Jeffrey S. , Klumpp Klaus 
Published: June 2022

e14550 Background: ENPP1 is a type II transmembrane protein with nucleotide pyrophosphatase and phosphodiesterase enzymatic activities. ENPP1 expression is associated with poor prognosis across cancer types. ENPP1 hydrolyzes extracellular cGAMP, nucleoside triphosphates (ATP, GTP) and dinucleotide polyphosphates. ENPP1 inhibition protects cGAMP and ATP and reduces adenosine levels in the tumor microenvironment, activates antigen-presenting cells and increases T-cell infiltration promoting anticancer immunity. RBS2418 is a potent oral inhibitor of ENPP1 enzyme activity and under evaluation in a Phase 1a/b study in subjects with advanced unresectable, recurrent or metastatic tumors as monotherapy and in combination with pembrolizumab. Methods: Expanded access to RBS2418 was granted by the FDA under a single patient IND to a subject with high grade adreno-cortical carcinoma. The subject received pembrolizumab at 200mg IV every 3 weeks with escalating doses of RBS2418 weekly, starting at 100 mg p.o. BID, followed by 200 and 400 mg p.o BID. Blood samples were collected at each dose level to determine plasma RBS2418 concentration and the corresponding serum ENPP1 level inhibition. Pharmacokinetic data supported a fixed dose of 200mg p.o. bid. Safety and target engagement were closely monitored and protein expression profiling by IHC in the pre-treatment tumor sample was done. Results: All evaluated dose levels of RBS2418 in combination with pembrolizumab were found to be safe and well tolerated. No dose-limiting drug-related toxicities were observed. The plasma concentration of RBS2418 was 29 ng/ml at 12 hours after the second 100 mg dose (Ctrough). This concentration corresponded to approximately 17-fold times the 90% inhibition level of ENPP1 in human serum (17x serum EC90). After the first dose of 200 mg RBS2418 a maximal plasma concentration of 473 ng/ml was measured, while the Ctrough at 12 hours after the second 200 mg dose was 69 ng/ml (corresponding to ̃40x serum EC90). There was ENPP1, but no detectable cGAMP in the baseline serum sample. In serum samples collected during treatment with RBS2418 there was no detectable ENPP1 enzyme activity and cGAMP was fully stable. Conclusions: RBS2418 with pembrolizumab was safe and well tolerated and no drug-related safety concerns were reported. The pharmacokinetics of RBS2418 showed excellent oral bioavailability, with plasma levels of RBS2418 > 17-fold and > 40-fold above serum EC90 of ENPP1 at the 100 and 200 mg BID dose levels, respectively. Dose levels are further studied in the current Phase 1 trial. ENPP1 activity was measurable in the serum of this subject and RBS2418 led to complete enzyme inhibition throughout the study at all dose levels investigated. These results support the continued development of RBS2418, the first member of this novel class of immunotherapy agents.