Comparison of Chemotherapeutic Activities of Rhodamine-Based GUMBOS and NanoGUMBOS

Rhodamine derivatives have been widely investigated for their mitochondrial targeting and chemotherapeutic properties that result from their lipophilic cationic structures. In previous research, we have found that conversion of Rhodamine 6G into nanoGUMBOS, i.e., nanomaterials derived from a group of uniform materials based on organic salts (GUMBOS), led to selective chemotherapeutic toxicity for cancer cells over normal cells. Herein, we investigate the chemotherapeutic activity of GUMBOS derived from four different rhodamine derivatives, two bearing an ester group, i.e., Rhodamine 123 (R123) and SNAFR-5, and two bearing a carboxylic acid group, i.e., rhodamine 110 (R110) and rhodamine B (RB). In this study, we evaluate (1) relative hydrophobicity via octanol–water partition coefficients, (2) cytotoxicity, and (3) cellular uptake in order to evaluate possible structure–activity relationships between these different compounds. Intriguingly, we found that while GUMBOS derived from R123 and SNAFR-5 formed nanoGUMBOS in aqueous medium, no distinct nanoparticles are observed for RB and R110 GUMBOS. Further investigation revealed that the relatively high water solubility of R110 and RB GUMBOS hinders nanoparticle formation. Subsequently, while R123 and SNAFR-5 displayed selective chemotherapeutic toxicity similar to that of previously investigated R6G nanoGUMBOS, the R110 and RB GUMBOS were lacking in this property. Additionally, the chemotherapeutic toxicities of R123 and SNAFR-5 nanoGUMBOS were also significantly greater than R110 and RB GUMBOS. Observed results were consistent with decreased cellular uptake of R110 and RB as compared to R123 and SNAFR-5 compounds. Moreover, these results are also consistent with previous observations that suggest that nanoparticle formation is critical to the observed selective chemotherapeutic properties as well as the chemotherapeutic efficacy of rhodamine nanoGUMBOS.