Effects of N-(4-hydroxyphenyl)retinamide supplementation on vitamin A metabolism.

The efficacy of the retinoid N-(4-hydroxyphenyl)retinamide (4-HPR) has been demonstrated in the inhibition of cancers in a variety of tissues. Moreover, toxicity effects following administration of 4-HPR have been found to be reduced or absent when compared to other retinoids. Pharmacokinetic studies in both animals and humans have focused on the metabolism of 4-HPR and its metabolites, and relatively little information has been published detailing the effects of long-term administration of 4-HPR upon normal endogenous vitamin A metabolism. Thus, the present study was carried out to examine the effects of long-term administration of 4-HPR upon plasma and tissue vitamin A kinetics. Male Sprague-Dawley rats were fed either a control diet sufficient in vitamin A [CON group; 1.0 retinol (ROH) equivalents/g diet] or a CON diet supplemented with 4-HPR (CON+4HPR group; 1173 micrograms 4-HPR/g diet). Following i.v. injection of a physiologically radiolabeled dose of ROH, ROH tracer and tracee kinetics were monitored in plasma and tissues over a 41-day period. Kinetic parameters were determined using the SAAM/CONSAM computer modeling programs to carry out graphical analysis of the tracer concentration curves. Mean plasma ROH levels measured for the CON+4HPR group were reduced to one-third of those of the CON group. Most of the kinetic parameters calculated were found to be significantly altered by the inclusion of 4-HPR in the diet. The fraction of the plasma ROH being catabolized per day (fractional catabolic rate) was nearly twice as high in the CON+4HPR treated group (3.61 +/- 0.49 day-1; mean +/- SD) as compared to the CON group (2.00 +/- 0.68 day-1). The amount of time that vitamin A molecules spent in the body before being lost irreversibly from the system (system residence time) was decreased by half in the CON+4HPR group (19.20 +/- 7.13 days) versus the CON group (38.63 +/- 9.62 days). Despite the increased catabolic rates and decreased system residence times measured for the CON+4HPR group, the estimated vitamin A use in these animals (11.01 +/- 3.10 micrograms/day) was 33% less than that used by the CON group (16.31 +/- 2.47 micrograms/day). Studies investigating the mechanisms by which 4-HPR alters vitamin A kinetics are presently under way in our laboratory. Nevertheless, these results suggest that long-term administration of 4-HPR markedly perturbs normal vitamin A metabolism in rats. Whether 4-HPR similarly alters human vitamin A metabolism with untoward clinical consequences deserves careful evaluation.