Abstract PO1-05-08: Enhanced ER+ tumor growth inhibition of fulvestrant from effective oral delivery yielding elevated plasma concentrations

Abstract Fulvestrant has been a backbone of endocrine therapy (ET) for ER+/HER2- metastatic breast cancer (mBC) for over 20 years due to its nanomolar potency, purely antagonistic behavior, and well-tolerated adverse event (AE) profile. It remains the only selective estrogen receptor degrader (SERD) approved in ER+/HER2- mBC regardless of ESR1 mutation status. While ESR1 mutations reduce fulvestrant binding affinity, they do not implicitly lead to acquired resistance as with aromatase inhibitors (AIs) and can be overcome by increasing exposure.1 Further, preclinical evidence suggests that higher exposure to fulvestrant correlates with higher efficacy in rodents.2 Unfortunately, fulvestrant is currently delivered at its maximum feasible dose due to its poor solubility and a lack of effective drug delivery technologies. In response, several novel oral SERDs have been brought into clinical development to improve upon the efficacy of fulvestrant. However, multiple have failed to prove superior to fulvestrant in its intramuscular (IM) injection formulation and improvements from elacestrant (EMERALD) and camizestrant (SERENA-2) are largely attributed to patients with ESR1 mutations after prior ETs. Therefore, enabling higher exposure of fulvestrant could prolong progression-free survival and improve quality of life for ET-naïve mBC patients and those that have progressed on ETs. VeraMorph has leveraged a proprietary oral drug delivery technology to overcome the limitations of fulvestrant’s poor solubility and potentially help mitigate acquired resistance by improving exposure via daily oral dosing. The oral dosages, which consist of a novel polymer-based soft gummy, stabilize fulvestrant in simulated intestinal media up to 5 mg/mL without influencing fulvestrant permeability or causing cytotoxic effects. Pharmacokinetic (PK) studies of oral fulvestrant in rodents have demonstrated maximum plasma concentrations up to 450 ng/mL, a level roughly 20 times that of the average exposure from the monthly IM formulation in clinical settings. PK studies also demonstrated that oral exposure increased with a roughly dose-proportional relationship, with a reduction in AUC from day 1 to day 8 upon repeat dosing. In a cell-derived xenograft (CDX) study utilizing ER+ MCF-7 tumors implanted in athymic nude mice, once daily dosing via oral gavage of oral fulvestrant at 125 mg/kg was able to achieve a statistically significant 55% reduction in tumor growth by day 42 relative to a 50 mg/kg dose of the IM formulation delivered subcutaneously once per week, which was determined to create an equivalent exposure to the monthly IM human dose in separate studies. Oral fulvestrant yielded no observed adverse effects up to 125 mg/kg in both the 42-day CDX study in mice or an 8-day safety study in rats. Further, onset of action and acquired resistance are being tested by monitoring cell viability (via the MTT assay) with MCF-7 cells exposed to time profiles of fulvestrant exposure resembling oral and monthly intramuscular delivery. The study demonstrated that oral dosing reaches minimal cell viability 1-2 weeks faster than IM dosing. Also, preliminary data suggests that the onset of acquired resistance (as demonstrated by a resumption of cell growth) may occur faster from the steady exposure of IM dosing relative to the modulation of daily oral dosing. These results suggest that oral fulvestrant has the potential to finally realize the full potential of fulvestrant as an effective ET for metastatic breast cancer with a well-tolerated AE profile, enhanced efficacy, and reduced susceptibility to acquired resistance with applicability across all mBC patients. 1 Brett JO, Spring LM, Bardia A, Wander SA (2021) Breast Cancer Research, 23(1):1–15. 2 Wardell SE, et. al. (2020) Breast Cancer Research and Treatment, 179(1):67–77. Citation Format: Doug Godfrin, Matthew Butchbach. Enhanced ER+ tumor growth inhibition of fulvestrant from effective oral delivery yielding elevated plasma concentrations abstract. In: Proceedings of the 2023 San Antonio Breast Cancer Symposium; 2023 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2024;84(9 Suppl):Abstract nr PO1-05-08.