Abstract RF01-02: A randomized Phase I pre-operative window trial of transdermal endoxifen in women planning mastectomy: evaluation of dermal safety, intra-mammary drug distribution, and biologic effects

Abstract Background: Breast cancer prevention requires only local exposure of the breast to active drug but oral preventive agents entail systemic exposure, with resultant adverse effects, limiting acceptance by high-risk women. Drug-delivery through the breast skin is an attractive option, but requires demonstration of dermal safety and drug distribution throughout the breast. We formulated the tamoxifen metabolite (E/Z)-endoxifen in a hydroalcoholic/oleic acid gel for transdermal delivery. We tested this in a placebo-controlled, double-blinded Phase I trial with dose escalation from 10 to 20 mg daily and evaluated dermal safety, intra-mammary drug distribution, and biologic effects in DCIS or invasive cancers. Methods: Women planning mastectomy for therapy for Stage 0-III breast cancer, or surgical risk reduction (N=32) were randomized (2:1) to placebo-gel or endoxifen-gel applied to the skin of both breasts for 3-5 weeks. Our primary endpoint, dermal safety was evaluated using the modified NCI Common Terminology Criteria for Adverse Events version 4. Endoxifen concentration, and modulation of circulating tamoxifen biomarkers were evaluated to demonstrate minimal systemic effects. Plasma and tissue concentration of endoxifen were determined using Liquid Chromatography – Tandem Mass Spectrometry. Tamoxifen-responsive circulating markers, insulin-like growth factor 1 (IGF1) and sex hormone-binding globulin (SHBG), were measured with enzyme-linked immunosorbent assays. Finally, we explored anti-cancer efficacy in tumors by measuring the modulation of tumor cell proliferation (Ki67 labeling index) and gene expression associated with tamoxifen response in proliferation (BIRC5, MKI67, MYBL2, CCNB1, AURKA), cell cycle (UBE2C, PLK1, CEP55, CDK1, RRM2, TOP2A, PTTG1), cancer-associated stromal genes of invasion (FN1, SERPINH1, PLOD2, PDGFA, ITGAV) and extracellular matrix (COL11A1, SPP1, COMP, BGN). Within-participant changes between baseline and post-treatment were assessed using the Wilcoxon signed rank test. Differences between treatment arms were assessed using the Wilcoxon rank sum test and the Kruskal-Wallis test. Results: endoxifen-gel at both doses incurred no dermal or systemic toxicity nor change in circulating biomarkers (IGF1, SHBG) compared to placebo. All treated breasts contained endoxifen at each of five sampling locations; the median per-person tissue concentration in the treated participants was 0.6 ng/g (IQR 0.4-1.6), significantly higher (p 0.001) than the median plasma concentration (0.2 ng/mL, IQR 0.2-0.2). The median ratio of the more potent (Z)-isomer to (E)-isomer at each breast location was 1.5 (IQR 0.96-2.54, p 0.05). We observed non-significant overall reduction in tumor Ki67 LI (median change -1.7 with IQR -5.8, 3.6) and gene expression of MKI67 and the genes associated with proliferation, and cell cycle. However, downregulation of invasion signature genes (FN1, SERPINH1, PLOD2, PDGFA, ITGAV) was significant (p=0.03) and a cluster of six participants demonstrated a consistent reduction of MKI67 and multiple gene signatures suggesting the possibility that their tumors might be early responders to low endoxifen exposure within 3 to 4 weeks of treatment. Conclusions: our evaluation of transdermal (E/Z) endoxifen 10 and 20 mg in hydroalcoholic/oleic acid formulation demonstrates lack of early dermal toxicity, and good tolerability over a month or so of therapy. Our findings warrant development of transdermal formulations of (Z)-endoxifen with better dermal permeation for a Phase 2 breast cancer prevention trial compared to low-dose oral SERMs so that women at high-risk for breast cancer may have a range of cancer prevention options available to them. Citation Format: Oukseub Lee, Latifa Bazzi, Yanfei Xu, Erik Pearson, Minhua Wang, Omid Hosseini, Azza Akasha, Jennifer Choi, Masha Kocherginsky, Kelly Benante, Thomas Helland, Gunnar Mellgren, Eileen Dimond, Marjorie Perloff, Brandy Heckman-Stoddard, Seema Khan, Scott Karlan, Melissa Pilewskie. A randomized Phase I pre-operative window trial of transdermal endoxifen in women planning mastectomy: evaluation of dermal safety, intra-mammary drug distribution, and biologic effects 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 RF01-02.