PCS6422

On August 23, 2020, we entered into a License Agreement ("Elion License Agreement") with Elion Oncology, Inc. ("Elion"), pursuant to which we acquired an exclusive contingent license to develop, manufacture and commercialize PCS6422 globally.

Elion acquired the eniluracil (PCS6422) product from Fennec Pharmaceuticals (formerly known as Adherex Technologies) in 2016. PCS6422 is an oral, potent, selective, and irreversible inhibitor of dihydropyrimidine dehydrogenase (DPD), the enzyme that rapidly metabolizes 5-FU, a common chemotherapy drug, to inactive metabolites, such as α-fluoro-β-alanine (F-Bal). F-Bal is thought to cause the neurotoxicity and Hand–Foot Syndrome (HFS) associated with 5-FU, and greater formation of F-Bal appears to be associated with a decrease in the antitumor activity of 5-FU. HFS can affect daily living activities, quality of life, and requires dose interruptions/adjustments and even therapy discontinuation resulting in suboptimal tumor effects. We believe that the inhibition of DPD by PCS6422 may significantly improve exposure to 5-FU and reduce 5-FU side effects related to F-Bal. One dose of PCS6422 irreversibly blocks DPD activity for up to two weeks until DPD levels recover via de novo synthesis of the DPD enzyme. Thus, we believe inhibition of tumor DPD will result in higher 5-FU intra-tumoral concentration and potentially better tumor response along with the decrease in F-Bal.

Fluoropyrimidines (e.g., 5-FU) are still the cornerstone of treatment for many different types of cancers, either as monotherapy or in combination with other chemotherapy agents by an estimated two million patients annually. Xeloda®, an oral pro-drug of 5-FU, is approved as first-line therapy for metastatic colorectal and breast cancer. However, its use is limited by adverse effects such as the development of HFS in up to 60% of patients.

Elion evaluated the potential for the combination of PCS6422 with capecitabine (Xeloda®, and, together with PCS6422, known as ECAPE) as a treatment of advanced gastrointestinal (GI) tumors. Nonclinical efficacy data indicated that in colorectal cancer models, pretreatment with PCS6422 enhanced the antitumor activity of capecitabine. PCS6422 increased the antitumor potency of capecitabine while not increasing the toxicity. The antitumor efficacy of the combination of PCS6422 and capecitabine was tested in several xenograft animal models with human breast, pancreatic and colorectal cancer cells. These preclinical xenograft models demonstrate that PCS6422 potentiates the antitumor activity of capecitabine and significantly reduces the dose of capecitabine required to be efficacious.

Elion met with the FDA in 2019 and agreed upon the clinical development program required for the combination of PCS6422 and capecitabine as first-line therapy for metastatic colorectal cancer when treatment with fluoropyrimidine therapy alone is preferred. Subsequently, an IND has been granted safe to proceed by FDA on May 17, 2020, for the Phase 1B study. This Phase 1B study will evaluate the safety and tolerability of several dose combinations of PCS6422 and capecitabine in advanced GI tumor patients and should be initiated in the first half of 2021.

Other DPD enzyme inhibitors (e.g. Gimeracil used in Teysuno® approved only outside the US) act as competitive reversible inhibitors. These agents must be present when 5-FU or capecitabine are administered to inhibit 5-FU breakdown by DPD in order to improve the efficacy and safety profiles of 5-FU. Given the reversible nature of their effect on DPD, over time 5-FU metabolism to F-Bal will return, decreasing the amount of 5-FU in the cancer cells and decreasing the potential cytotoxicity on the cancer cells. There is also evidence that administering DPD inhibitors directly with 5-FU may also decrease the antitumor effect of the 5-FU. Because PCS6422 is an irreversible inactivator of DPD, it can be dosed the day before capecitabine administration and its effect on DPD can last longer than the reversible DPD inhibitors and beyond the time 5-FU exists in the cancer cell. We believe this can optimize the potential cytotoxic effect and minimize the metabolism of 5-FU.

Prior to Elion’s involvement, two multicenter Phase 3 studies were conducted in patients with colorectal cancer (CRC) with PCS6422 administered in 10-fold excess to 5-FU. Unfortunately, we believe the dose of PCS6422 during these trials was not optimal, and that PCS6422 was not administered early enough to irreversibly affect the DPD enzyme, thus the regimen tended to produce less antitumor benefit than the control arm with the standard regimen of 5-FU/leucovorin (LV) without PCS6422. Later preclinical work suggested that when PCS6422 was present at the same time as and in excess to 5-FU, it diminished the antitumor activity of 5-FU, which we believe supports the proposal of exploring clinically dosing PCS6422 several hours before 5-FU to allow its clearance before the administration of 5-FU.