Quaternary Phosphonium Salts Outperformed Vemurafenib (PLX) and Etoposide Against BRAFV600D,V600E PLX-Resistant Melanoma and MDR Neuroblastoma, Exhibiting No/Low Toxicity on 3T3/HaCaT Cells

Late-stage metastatic cutaneous melanoma (MCM) and neuroblastoma (NB) are the most aggressive skin and childhood cancers with survival rates of <50%, mainly due to the emergence of resistance to available drugs, thus requiring an urgent solution. Quaternary phosphonium salts (QPSs) can exhibit strong anticancer effects, regardless of the developed resistance. Triphenyl (1) and diphenyl (3 and 4) phosphonium salts were synthesized, treating commercial triphenyl phosphine and synthesizing 11-diphenylphosphanyl-undecan-1-ol (2), respectively, with benzyl bromide. Upon full characterization, they were tested, for the first time, on MeTRAV (BRAFV600D) and MeOV (BRAFV600E) vemurafenib (PLX)-resistant MCM cells, etoposide (ETO)-sensitive (HTLA 230) and multidrug resistant (MDR) (HTLA ER) NB cells, non-tumorigenic human keratinocytes (HaCaT), and mouse embryonic fibroblasts (3T3), as well as red blood cells (RBCs). Viability of MeTRAV cells was decreased to 44.8% by administration of 1 (100 µM), in intermediate-time (48 h) treatments, while short-time exposure (24 h) to 3 (≥75 µM) and 4 (≥50 µM) was sufficient to reduce their viability to 33.6 and 32.2%. Viability of MeOV was decreased under 50% with 5 µM concentrations of 1 and 25 µM of 3 and 4, While cells were exterminated (26.9, 20.6, and 21.8%) with higher concentrations after 48 h exposure. Collectively, 1 was the better performing compound (IC50 = 6.4 µM, 48 h). Viability of HTLA ER cells was decreased under 50% upon 72 h administrations of 1 at concentrations ≥ 50 µM, 48 h (≥75 µM) and 72 h (≥50 µM) of 3, and after 72 h (≥75 µM) of 4, but 72 h exposure and high concentrations of all compounds were necessary for their extermination (31.2, 28.7, and 29.7%). Viability of HTLA 230 cells was not <50% when 1 and 4 were administered for only 24 h, while their viability was <50% after administration of 3 at all times of exposure. At high concentrations, all compounds exterminated cells (33.6, 25.3%, 1, 48–72 h; 38.6, 30.2, and 24.7%, 3, 24–72 h; 33.2%, 4, 72 h). The best-performing compounds were 1 (IC50 = 4.0 µM, HTLA 230) and 3 (IC50 = 27.8 µM, HTLA ER) at 72 h exposure. The cytotoxic effects of compound 4 on MeTRAV cells, when exposed to 24/48 h treatments, were comparable to those of PLX on the same cells in 72 h treatments. Compound 1, in shorter 48 h treatments of PLX-R MeOV, was 2.5-fold more cytotoxic than PLX in 72 h ones. All compounds were not cytotoxic to 3T3 cells at all times of exposure; they had low cytotoxicity to HaCaT cells in 24 and 48 h treatments and were slightly cytotoxic to RBCs in 24 h ones. Compound 1 could be a promising platform to develop new intermediate-time therapies for PLX-R MeOV cells, while 4 could be used to develop 24 and 48 h treatments for PLX-R MeTRAV cells. Also, all compounds could be developed as new treatment options for both ETO-sensitive and MDR late-stage HR-NB cells, being even more effective than ETO by 1.2, 2.0, and 1.3 times (HTLA 230) and 3.2, 4.7, and 3.2 times (HTLA ER). All compounds have the potential to be developed as adjuvants in already existing anticancer cocktails to treat MCM and/or NB.