Thermally Generated Triplet Excited Ketones Mediated Photooxidation of Penciclovir

Objective: The objective of the present study to investigate photooxidation of PC in presence of thermally generated triplet excited ketones and in presence of photosensitizers rose bengal and riboflavin. Methods: The Photooxidation of antiviral drug penciclovir was studied under different reaction conditions. First we treated the penciclovir (PC 1) with triplet excited ketones, which have been generated in thermal decomposition of 3-(hydroxymethyl)-3, 4, 4-trimethyl-1, 2-dioxetane (HTMD), in the dark. Photosensitized oxidation of penciclovir was also carried out in presence of photosensitizers riboflavin and rose Bengal to compare the yield of photoproducts under both condititions. Results: Three major oxidation products were detected by means of spectroscopic measurements. The products were 4-hydroxy(3-Hydroxymethyl) butyl spiroiminodihydantion (2), 4-[4-hydroxy(3-hydroxymethyl)butylamino]-2-imino-1,2-dihydroimidazol-5-one(3), 2, 2-diamino-4[4-hydroxy-(3hydroxymethyl)butylamino]oxazol-5-one(4). Conclusion: The result of our present investigation reveals that triplet excite ketone generated by thermal decomposition of triplet excited ketone oxidize PC efficiently to the Sp, by a type II photooxidation mechanism and to the Ox and Im by a type I mechanism. Keywords; Penciclovir, photooxidation, 1,2dioxetane, triplet excited ketone. INTRODUCTION Indeed, despite their excellent therapeutic activity, many pharmacologically important chemicals such as antibacterials, antimicotics and non-steroidal anti-inflammatory drugs (NSAIDs) can induce phototoxic, photo allergic and photo mutagenic phenomena strictly related to the drug photochemical reactivity [1]. Recently the interest in the adverse photosensitization mechanism of various pharmacologically important chemical has been markedly intensified. The pharmacologically active drug shows photosensitization by two mechanisms either by type I mechanism called radical mediated or type II mechanism or singlet oxygen mediated. Many of the endogenous (e.g. flavins, tetrapyrrols, protoporphyrins etc.) or exogenous photo sensitizers can elicit phototoxic or photo allergic responses [2, 3]. Likewise there are many sensitizing dyes e.g. rose Bengal, benzophenone, riboflavin etc. that are activated by light. Triplet-excited ketones, important classes of photo oxidative sensitizer [4, 5] are of biological interest since they may be generated in cellular systems upon exposure of endogenous chromophores to the UV irradiation or by dark reactions (e.g. lipid per oxidation and enzymatic oxidation) [6]. Triplet excited ketones may be produced either by thermal decomposition of 1, 2-Dioxetanes or by their conventional photochemical generations [7]. These thermally generated triplet excited ketones are analogous to those produced photo chemically and operate as type I or type II photo oxidants [8]. 1, 2-Dioxetanes are unique class of four membered ring peroxide and are of biological interest, since they have been implicated as labile intermediates in oxidative stress [9]. Penciclovir is a nucleoside analogue that inhibits HSV1 and HSV2 similar to the acyclovir but has the advantage of prolonged half life in infected cells [10]. Penciclovir 1% has been introduced as an effective topical treatment of herpetic eruptions [11] independent of the stage of development of lesions. Penciclovir has similar in vitro efficacy and mechanism of action to acyclovir [12, 13]. In our recent study for the singlet oxygen mediated photooxidation of PC in aqueous solution, we have isolated spiroiminodihydantoin (sp, 2), imidazolone (Im, 3) and oxazolone (Ox,4) as the three major products [14]. Here in we have investigated photooxidation of PC in presence of thermally generated triplet excited ketones and in presence of photosensitizer’s rose bengal and riboflavin. The distribution of products were compared with those PC photooxidation by type I sensitizer, riboflavin and a type II sensitizer rose Bengal [15]. Waseem Ahmad / International Journal of Pharma Sciences and Research (IJPSR) ISSN : 0975-9492 Vol 6 No 12 Dec 2015 1418 EXPERIMENTAL Chemicals and Reagents All chemicals used were of analytical grade. Penciclovir was extracted from the commercial medicament denavir (dev chem., Mumbai, India) with a soxhlet extractor, purified by TLC and recrystallized from the same solvent. Melting point, H-NMR and Co-TLC with authentic pure sample determined the purity of penciclovir. HTMD was synthesized according to the literature procedure. Standard samples of spiroiminodyhydantoin, imidazolone and oxazolone were prepared by rose Bengal photosensitized oxidation of Penciclovir. OXIDATION PROCEDURE For the dioxetane mediated oxidation of Penciclovir (PC), a 0.5 mM solution of PC in10 mM sodium cacodylate buffer (pH 7.0) and 10 vol% dioxetane solutions in acitonitrile was kept at 50C for several hour in absence of light. The photosensitized oxidation of phosphate buffered solution of PC (0.5mM) was carried out in the presence of photosensitizers riboflavin and rose Bengal. A 150-W sodium lamp was used for carrying out photosensitized oxidation of PC with riboflavin and rose Bengal. The lamp was placed at 15cm below the bottom of the ice filled beaker, in which a round bottom flask having reaction mixture was placed. HTMDinduced photooxidation was carried out in dark for 18 hr at 50C by using acetonitrile (10%by vol) as co solvent while the photooxidation by riboflavin and rose Bengal was carried out for 2.5 hr. A number of products were indicated on TLC at the end of reaction, from which the only three major photoproducts 2, 3 and 4 were obtained in isolable yields. The amount of photoproducts formed and un-oxidized PC was assessed by isolation and purification of the phtolyst using silica gel column chromatography. Effects of D2O on the yield of Sp in HTMD-induced photooxidation were also observed to confirm the involvement of singlet oxygen. RESULT AND DISCUSSION The thermal HTMD mediated Photooxidation of PC at 50C Produces three major products (3-Hydroxymethyl) butyl spiroiminodyhydentoin (2), 4-[4-hydroxy(3-hydroxy methyl) butyl amino]-2-imino-1,2-dihydroimidazol5-one(3), 2, 2-diamino-4 [4-hydroxy-(3-hydroxymethyl) butyl amino] oxazol-5-one (4) (Scheme-1) . All the photoproducts were identified by comparing their spectral data and TLC with those of authentic pure samples of spiroiminodyhydentoin (Sp), imidazolone (Im) and oxazolone (Ox) generated in the rose Bengal mediated photooxidation of PC. The spectral data of all the three products were found to be in a close agreement with well established singlet oxygen mediated PC photooxidation products.