A study of the photochemical stability of 3'-Azido-3' deoxythymidine in different solvents.

Acquired immunodeficiency syndrome (AIDS) is a disease that causes weakening of the human immune system which is thus unable to fight off infections initiated by the human immunodeficiency virus (HIV). HIV/AIDS is the fourth leading cause of death in the world and South Africa has one of the highest HIV infection rates in the world, with approximately 6.2 million people infected. Zidovudine (AZT) [1-[(2R, 4S, 5S)-4-azido-5-(hydroxymethyl) oxolan-2-yl]-5-methylpyrimidine-2,4-dione] was the first HIV drug approved by the US Food and Drug Administration (FDA) to treat HIV-infected patients. Its molecule consists of three photoactive groups, namely, the alkene, carbonyl and azido groups. This study focusses on the investigation of the photochemical stability of AZT in different media. The photochemical stability of 3'-azido-3'-deoxythymidine (AZT) was investigated in different solvents, namely, water, methanol, ethanol, 2-propanol and acetonitrile. The samples were irradiated under different conditions of pH (3, 5 and 7) and concentration (5.01 × 10, 4.94 × 10 and 2.39 × 10 M). The photoreactions were monitored by ultraviolet and visible (UV/Vis) spectroscopy and reversed-phase high performance liquid chromatography (RP-HPLC). Acetophenone was used as the internal standard for quantification of AZT. Photodegradation products were identified by using liquid chromatography-mass spectrometry (LC-MS). The results show that the photodegradation of AZT is strongly dependent on the solvent, the pH of the solution as well as on the initial concentration. The UV/Vis spectra of AZT upon photolysis showed a hyperchromic effect both at 213 and 265 nm in all of the different solvents, pH and concentration conditions. However, the change in absorbance was inconsistent with respect to irradiation time. We proposed that this could be attributed to competitive absorption by the photoproducts. The hypsochromic effect was also observed ii under all conditions of pH. A new absorption band at 355 nm was formed in acetonitrile which was predicted to be the result of the elimination of nitrogen to generate a nitrene as a reactive intermediate, which may further undergo intermolecular interaction with the solvent to form new photoproducts. The photodegradation products of AZT retained almost the same retention time and UV spectra under all the different conditions of solvent, pH and concentration. This indicates that the photoproducts formed have the same chromophore as AZT, which we propose to be a thymine moiety. Three photoproducts were formed at pH 5 and 7 and two photoproducts were formed at pH 3. For the effect of the initial concentration, three photoproducts were formed at the intermediate concentration (4.94 × 10 M) in all the different solvents, investigated. Three photoproducts were also formed at the lowest concentration ( 2.39 × 10 M) in all solvents, except in ethanol (two photoproducts were observed). Only one photoproduct was formed at high concentration (5.01 × 10 M) in water and acetonitrile and two photoproducts were formed in alcoholic solutions (methanol, ethanol and 2-propanol). The HPLC peak area of AZT decreases with increase in irradiation time whereas the HPLC peak areas of the photoproducts increase with increase in irradiation time for all conditions of solvent, pH and concentration. The photodegradation of AZT occurs faster as the dipole moment of the solvent increases in order: methanol < ethanol < water < acetonitrile, except for 2-propanol. The rate of photodegradation of AZT is more rapid at pH 7 than at pH 3 and 5. For water, methanol and 2-propanol, the rate of photodegradation was higher for the lowest concentration followed by the intermediate and then the highest concentration. For ethanol and acetonitrile, the rate of photodegradation was higher at the intermediate concentration followed by the lowest and highest concentrations. The mass spectral results revealed that the azido group was accountable for the photoreactivity of the drug. Thymine was found to be the major iii photoproduct under all conditions of pH, solvent and concentration. Five photodegradation pathways were proposed to account for the photoproducts formed in protic and aprotic polar solvents.