Spectral properties and ionization behavior of retinoids, II

Superoxidation of retinoic acid

Atmospheric pressure chemical ionization mass spectroscopy (APCI-MS) was used to examine the light-induced oxidation products of retinoic acid under conditions that favor and preclude its aggregation. We observed that in conditions that favor aggregation, i.e. in aqueous solutions, retinoic acid undergoes superoxidation to yield highly oxidized species. Oxidation is limited, however, in the absence of such communication, i.e. when the polyene is fully solvated. From a comparison of the measured MS with that obtained from chemical oxidation of retinoic acid under conditions that promote radical oxidation and singlet oxygen-mediated oxidation, we conclude that superoxidation is mediated by reactive oxygen species other than singlet oxygen.

Acid dissociation constants of bilirubin and related carboxylic acid compounds in bile salt solutions

Bilirubin, the yellow-orange tetrapyrrole pigment of jaundice, is essentially insoluble in pure water, but is much more soluble in solutions of bile salts such as sodium taurocholate. The biophysical chemistry of bilirubin in bile salt solutions is affected by changes in the pH of the solution in the range 5-9, suggesting that interactions with bile salt molecules and micelles may alter the acidity of the pigment. We have examined this possibility by determining the apparent pKa values for a series of carboxyl 13C-enriched model compounds, including the bilirubin analog mesobilirubin XIIIalpha, in solutions of sodium taurocholate and sodium taurodeoxycholate. Apparent pKa values were determined by 13C NMR titrations in dimethyl sulfoxide-water mixtures. The results show that the acidity of all compounds is decreased, or pKa increased, in micellar bile salt solution relative to pure water and that the effect is greatest for the larger, less water-soluble compounds. We have proposed a model to explain these results and discussed the implications of these findings for the biophysical chemistry of bilirubin in bile.

2-hydroxypropyl-beta-cyclodextrin increases aqueous solubility and photostability of all-trans-retinoic acid

BACKGROUND

All-trans-retinoic acid (ATRA, vitamin A acid or tretinoin) is effective in the treatment of acute promyelocytic leukaemia (APL). Unfortunately, the oral absorption of ATRA is highly variable. Its poor aqueous solubility also makes it difficult to be formulated into parenteral formulation. To date, there is no parenteral formulation of ATRA available commercially.

OBJECTIVE

To undertake the preformulation work necessary for developing such a product.

METHOD

We investigated the solubility and stability profile of ATRA in various formulations.

RESULTS

The aqueous solubility of ATRA could be greatly increased by the inclusion of ATRA in 2-hydroxypropyl-beta-cyclodextrin (HP-beta-CD). Adjusting the pH value further improved the water solubility of ATRA. The photostability of HP-beta-CD-based formulation of ATRA was evaluated and it was found that inclusion ATRA into HP-beta-CD did improve the photostability of ATRA.

CONCLUSION

These results showed that it is possible to develop a parenteral formulation and/or an aqueous oral formulation of all-trans-retinoic acid by using 2-hydroxypropyl-beta-cyclodextrin. However, the biopharmaceutical properties of such a formulation would be necessary before its use.

Kinetic study of a 2-hydroxypropyl-beta-cyclodextrin-based formulation of all-trans-retinoic acid in Sprague-Dawley rats after oral or intravenous administration

all-trans-Retinoic acid (ATRA, vitamin A acid, or tretinoin) is a potent chemotherapeutic agent for the treatment of acute promyelocytic leukemia (APL). Its poor aqueous solubility not only affects its oral absorption but also prevents it from forming an aqueous parenteral formulation. Recently, we developed a water-soluble formulation of ATRA with 2-hydroxypropyl-beta-cyclodextrin (HPbetaCD). In present study, this formulation was tested in Sprague-Dawley rats. Kinetic study of ATRA was carried out after oral or intravenous administration. Though there were no statistical differences in any of the estimated pharmacokinetic parameters between ATRA sodium salt and HPbetaCD-based ATRA after intravenous administration, inclusion of ATRA into HPbetaCD was found to greatly improve the oral absorption of ATRA.