Melanocortin-4 Receptor Antagonism Inhibits Colorectal and Anaplastic Thyroid Cancer In Vitro and In Vivo

: Background/Objectives MC4R expression and its role in colorectal and anaplastic thyroid cancers, where resistance to therapy and lack of standard treatments remain significant challenges, are poorly understood. This study aimed to investigate MC4R as a potential therapeutic target in these cancers using the selective antagonist ML00253764 (ML), alone and in combination with vinorelbine (VNR) and irinotecan (or its active metabolite SN-38). Methods: Human colorectal adenocarcinoma HT-29, Caco-2, and anaplastic thyroid carcinoma 8305C cell lines were used. MC4R expression was assessed by Real-Time PCR with validated primers (Assay ID Hs00271877_s1), immunofluorescence, and Western blotting. Proliferation and apoptosis assays were conducted with ML, and synergy with VNR and SN-38 was evaluated by Combination Index and Loewe methods. ERK1/2 phosphorylation was measured using an ELISA assay. In vivo studies were conducted by injecting tumor cells into Athymic Nude-Foxn1nu mice, treated with ML, VNR, irinotecan, or their combinations. Results: MC4R expression was confirmed in all cell lines. ML treatment inhibited MC4R, producing antiproliferative and pro-apoptotic effects, with IC50 values of 7667 ± 2144.6 nM (8305C), 806.4 ± 321.8 nM (HT-29), and 2993 ± 1135.2 nM (Caco-2). In combination with VNR and SN-38, ML exhibited significant synergy in vitro and reduced tumor volume in vivo without causing weight loss or adverse effects in mice. Conclusions This study identifies ML as a promising therapeutic agent that, when combined with chemotherapy, may offer a novel strategy for treating colorectal and anaplastic thyroid cancers.