Twenty-two derivatives of retinoic acid (RA) were tested for their potency to induce in vitro morphological differentiation of SK-N-SH neuroblastoma cell line at concentration range of 0.1 nM to 10 muM. The results indicate that three derivatives Ro-13-4306; Ro-13-6307 and Ro-13-7410 were potent inducers of differentiation at 3-4 log lower concentration compared to all trans retinoic acid (ATRA). Other compounds such as Ro-08-8717 and Ro-40-6055, were totally inactive, and with the majority of compounds inducing differentiation at concentrations comparable to ATRA. Fourteen compounds were studied for their hydrophobicity properties and the empirical hydrophobicity index (EHI) was derived from the retention time on an HPLC reverse column. Most of the EIH values derived for the compounds tested fell within the expected differentiation potency (ID50) range. However, ATRA and 9-cis RA fell below the expected curve, i.e. were less active than predicted by their hydrophobicity properties, whereas Ro-13-6307 and Ro-13-4306 fell above the curve, and thus were more potent in inducing differentiation than was predicted from their hydrophobicity properties. Energy minimized molecular models for the theoretical crystal structures were also reconstructed by computer modelling for five compounds. The molecular models of the two most active compounds (13-4306 and 13-6307) had almost identical crystal structure which was slightly different from that of the less active compounds, ATRA and 13-7410. SK-N-SH cell line expressed mRNA transcripts for the nuclear RA receptor-alpha (RARalpha) and did not express the RARbeta or RARgamma receptor. Induction of differentiation with ATRA, or with Ro-13-7410 did not change the pattern of expression of mRNA transcripts of any of the nuclear receptors tested. However, 13-4306 and 13-6307 markedly reduced the expression of the 2.4 kb mRNA band of RAR-alpha. These results, taken together, suggest the increased potency of 13-6307 and 13-4306 might be the result of a different splicing patterns of the RAR-alpha mRNA transcripts.

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