Oxidation of HMG-CoA reductase inhibitors by tert-butoxyl and 1, 1-diphenyl-2-picrylhydrazyl radicals: model reactions for predicting oxidatively sensitive compounds during preformulation

Understanding Concomitant Physical and Chemical Transformations of Simvastatin During Dry Ball Milling

The present study investigates concomitant processes of solid-state disordering and oxidation of simvastatin during milling. The separate dry ball milling of crystalline and amorphous powders of simvastatin were conducted at ambient temperature for 10 and 60 min each. The relative crystallinity was determined using X-ray scattering and oxidative degradation was analyzed using liquid chromatography. The physical and chemical transformations in the milled powder were evaluated using modulated differential scanning calorimetry (DSC) and Fourier transform infrared spectroscopy. The disordering during milling of the crystalline powder was found to progressively decrease the crystallinity. For the amorphous starting material, milling for 10 min induced a large extent of recrystallization, while milling for 60 min largely re-amorphized the powder. This solid-state disordering and/or ordering were accompanied by progressive air oxidation during milling. The infrared spectroscopic analysis revealed the molecular manifestations associated with the physicochemical transformations in the disordered solid states. The melting point of simvastatin depressed systematically with the increase in the degree of disorder as well as the degradation. The in situ cooling in DSC of milled samples from their molten state led to the formation of the co-amorphous phase between the drug and degradation products, which showed a consistent increase in glass transition temperature with the increase in the content of degradation products. The study overall demonstrates the solid-state re-ordering and disordering of crystalline and amorphous simvastatin accompanied by chemical degradation as the consequence of the mechano-activation.

Gas-phase transfer of polymer cross-linking agents and by-products to solid oral pharmaceuticals

In the pharmaceutical industry, solid oral compressed tablets (OCT) are frequently transported in bulk containers prior to packaging. While in this state, the product is generally protected from interaction with liquid and solid contaminants by physical barriers (e.g., polyethylene bags, drums, etc.). Vapor phase contamination, although generally less frequently observed, is possible. A specific example of the detection and identification of volatile by-products (acetophenone and 2-phenyl-2-propanol) of a common polymer cross-linking agent (dicumyl peroxide) is presented. The product tablets were compressed, placed into double polyethylene bags, and subsequently placed into a polyethylene drum for shipment overseas. To cushion the product during transit, a cross-linked polyethylene foam disk (designed to fit into the bottom of the drum) was placed below the bag of tablets. Initially, these contaminants were detected by HPLC with UV detection at the receiving laboratory, and assumed to be degradates of the active components of the product. Further analysis showed that neither the collected UV absorbance data nor the observed levels of the contaminants were consistent with known degradates of the product. Liquid extraction followed by GC-MS analysis of the product as well as the cross-linked foam disk exhibited measurable quantities of the contaminants in question. Vapor phase transfer of these cross-linking agent by-products, originating in the cross-linked foam pads, was determined to be the root cause for the presence of these compounds in the product.

A Novel peroxy Radical Based Oxidative Stressing System for Ranking the Oxidizability of Drug Substances

A novel oxidative stressing system is described which generates high levels of peroxy radicals in solution at room temperature, without the use of azonitrile initiators. The oxidative stressing system is composed of a 10% solution of Tween 80 in water to which FeCl3 x 6H2O is added. The Tween 80 acts as a solubilizing agent for drug compounds, and also contains substantial amounts of organic hydroperoxides. It is shown that the Fe III/ Fe II couple operates on the hydroperoxide concentration to effectively generate new peroxy radicals, which then propagate in the Tween 80 solution. Key features of the Tween 80/Fe III system are investigated, and the oxidizability of seven known compounds and ten developmental compounds are examined. Relative reaction rates span a 300-fold range, from benzoic acid (nonreactive, defined as <0.5% reacted per day) to Vitamin D3 (7% reacted per hour). Oxidizability "rankings" thus generated are shown to agree well with azonitrile initiated oxidative stress. The potential for general correlations between this type of oxidizability data and actual oxidative performance in LFC and solid oral dosage forms is discussed.

Use of 2,2'-azobis(2-amidinopropane) dihydrochloride as a reagent tool for evaluation of oxidative stability of drugs

PURPOSE

To study the oxidative degradation of drugs using a hydrophilic free radical initiator, 2,2'-Azobis(-amidinopropane) dihydrochloride (AAPH).

METHODS

AAPH was used as the free radical initiator to study oxidation of three model compounds (A, B, and C), which represent different oxidizable moieties. In the solution model, the drugs and AAPH were dissolved in a mixture of acetonitrile and aqueous buffer and incubated at elevated temperatures to evaluate oxidative degradation. The effects of pH and drug-AAPH ratio on the kinetics of the reaction were evaluated for compound A. Commonly used antioxidants were also evaluated by addition to solutions of drug and AAPH. In the solid-state model, blends of drug with microcrystalline cellulose were treated with AAPH and placed at elevated temperature and humidity to evaluate solid state oxidation.

RESULTS

Use of AAPH resulted in selective oxidation of the model drugs by a free radical initiated process. The scope of the technique was further investigated in detail using compound A. The rate of oxidation of compound A varied directly with the concentration of AAPH. The pseudo first-order rate constants for the oxidative degradation were calculated from the kinetic data. The antioxidants were rank-ordered based on their quenching activity on the rates of AAPH initiated oxidation for compound A. The concept was extended to oxidation in solid state.

CONCLUSIONS

The proposed AAPH model is useful in assessing oxidative stability of drug candidates in development.

Topical Delivery of Retinyl Ascorbate Co-Drug

Retinyl ascorbate (RA-AsA), an ester co-drug of vitamins A (RA) and C (AsA), is proposed as a topical antioxidant/cell division regulator for reducing UV-induced generation of free radicals and disrupted dermal cell growth. The efficacy of dermatological agents is influenced by their retention within the skin, which is increased by the interaction with skin components. Keratin is the major protein (approximately 95%) in the skin, and this paper reports the binding of RA-AsA, RA, AsA, retinol, ascorbic acid palmitate and retinol palmitate to three tissues-human callus, pig ear skin and bovine horn keratin. Tissue samples were incubated with solutions of compounds and the uptake measured as the ratio of bound/free compound at equilibrium. Binding to keratin was assessed using delipidised tissue, and was much higher for the polar compounds, suggesting dipolar/H-bonding interaction. Binding strength was ranked as human > porcine > bovine, but there was no distinction for highly lipophilic compounds. The binding characteristic of native tissues was complicated by lipid content of the tissues. There seemed to be a dual effect. The binding of very lipophilic materials increased with lipid content, implying that a substantial amount is dissolved in the lipid matrix. For highly polar AsA, lipid content decreased the binding, suggesting that the lipid reduced the strong polar interactions with skin protein/keratin.

Chemistry of the t-butoxyl radical: evidence that most hydrogen abstractions from carbon are entropy-controlled

Absolute rate constants and Arrhenius parameters for hydrogen abstractions (from carbon) by the t-butoxyl radical ((t) BuO.) are reported for several hydrocarbons and tertiary amines in solution. Combined with data already in the literature, an analysis of all the available data reveals that most hydrogen abstractions (from carbon) by (t) BuO. are entropy controlled (i.e., TdeltaS > deltaH, in solution at room temperature). For substrates with C-H bond dissociation energies (BDEs) > 92 kcal/mol, the activation energy for hydrogen abstraction decreases with decreasing BDE in accord with the Evans-Polanyi equation, with alpha approximately 0.3. For substrates with C-H BDEs in the range from 79 to 92 kcal/mol, the activation energy does not vary significantly with C-H BDE. The implications of these results in the context of the use of (t) BuO. as a chemical model for reactive oxygen-centered radicals is discussed.

The role of hydrogen bonding on the h-atom-donating abilities of catechols and naphthalene diols and on a previously overlooked aspect of their infrared spectra

Catechols and 1,8-naphthalene diols contain one "free" hydroxyl and one intramolecularly H-bonded hydroxyl group. The "free" hydroxyls are strong hydrogen-bond donors (HBDs) with alpha2H values (Abraham et al. J. Chem. Soc., Perkin Trans. 2 1989, 699) ranging from 0.685 to 0.775, indicating that these compounds have similar HBD properties to those of strongly acidic phenols such as 4-chlorophenol (alpha2H = 0.670) and 3, 5-dichlorophenol (alpha2H = 0.774). Kinetic effects on H-atom abstractions from the diols in HB acceptor (HBA) solvents can be quantitatively accounted for over at least 50% of the available range of solvent HBA activities (as measured by their beta2H values; see Abraham et al. J. Chem. Soc. Perkin Trans. 2 1990, 521) on the basis of a single reactive OH group, the "free" OH. This free OH group is an outstanding H-atom donor in poor HBA solvents; e.g., in hexane rate constants for reaction with the DPPH* radical are 2.1 x 104 M-1 s-1 for 3,5-di-tert-butyl catechol and 2 x 106 M-1 s-1 for 4-methoxy-1,8-naphthalene diol, but only 7.4 x 103 M-1 s-1 for alpha-tocopherol (vitamin E). The diols are much more reactive than simple phenols because the O-H bond dissociation enthalpy of the "free" OH group is weakened by 5-9 kcal/mol by the intramolecular H-bond. The IR spectra of all the diols in CCl4 show two fairly sharp O-H stretching bands of roughly equal intensity separated by 42-138 cm-1. Addition of a low concentration of DMSO, a strong HBA, causes the band due to the intramolecularly H-bonded OH group to decrease in intensity to roughly half the extent that the "free" OH band loses intensity. The latter forms an intermolecular H-bond with the DMSO, the former does not. What has been overlooked in earlier work is that as the DMSO concentration is increased the band due to the intramolecularly H-bonded OH group first broadens and then evolves into a new, lower frequency (by 19-92 cm-1) band. The magnitude of the shift in the frequency of the intramolecular OH band caused by H-bonding of HBAs to the "free" OH group, Deltanu, increases linearly as the HBA activity of the additive increases, e.g., for 3,5-di-tert-butylcatechol, Deltanu/cm-1 = 33.8 beta2H (R 2 = 0.986). This may provide a new and simple method for determining beta2H values.