Solid-state photodegradation

One attractive possibility is the use of the a-keto-oxime chromophore. It has a strong absorption at 220 nm whose tail, which extends to 240-250 nm, would improve the absorption characteristics of PMMA. Also, the esters possess a N-O bond which is photochemically labile yet sufficiently thermally stable so as to be compatible with the various processing steps. The solution degradation of a-keto oximino methacrylate esters upon irradiation with light of X365 nm has been reported by Delz-enne (2), and we proceeded to investigate the solid state photodegradation of similar copolymers and their possible utility as deep UV photoresists. 

Various techniques have been applied in studies of solid samples. A direct method based on infrared spectroscopy has been used in the study of pharmacologically active compounds such as nitroimidazole derivatives (chemotherapeutics) and nifedipine (Marciniec and Rychcik, 1994 Marciniec et al., 1997). This approach can be applied in kinetic and quantitative examinations of the degradation process. An alternative method based on diffuse reflectance spectrophotometry takes into account the surface reflectance of the sample (Zhan et al., 1995). In recent years, Raman spectroscopy has become a powerful tool for the study of various processes in the solid state. The technique allows mapping of the concentration of one specific component within a sample, e.g., an active ingredient in a tablet formulation. This method may therefore become a useful tool in future studies of solid-state photodegradation (Opel and Venturini, 2002). 

Miller B L, Hageman M J, Thamann T J, et al. (2003). Solid-state photodegradation of bovine somatotropin (bovine growth hormone) evidence for tryptophan-mediated photooxidation of disnlfide bonds. J. Pharm. Sci. 92 1698-1709. 

B. Solid-State Photodegradation of a Substituted 1,2,4-Triazolo[4,3-a]pyridine... 

This is a preliminary approach to the use of a new generation of solid-state sensors based on the capacity of the sensor element to catalyze the photodegradation of various kinds of organic compounds and to recognize their structure on the basis of the type of process catalyzed. The electron holes present in the Ti02 structure are able to promote the oxidative process of substances present in the environment, in particular the ones easily adsorbed on it. Titanium dioxide is a well-known photocatalyst [5-13]. Less famous are its characteristics as sensor material [14-18] of the ability of the organic molecules to be completely degraded, that is mineralized. 

On the other hand, many excipients can act to chemically stabilize an API in the solid state and in solid dosage forms. The most common class of stabilizing excipients is cyclodextrins (36). Cyclodextrins can envelop the API in their hydrophobic cavities and shield it from common degradation reactions such as hydrolysis, oxidation, or photodegradation. Some excipients or additives may also act as complexing agents that provide hydrolytic (37) and oxidative (38) stabilization. Many excipients, such as cyclodextrins, dyes, and colored additives, are capable of providing extensive photostabilization in the solid state (39-41). 

The biphasic kinetics curve for the reactions of polymers is very typical and is found frequently in the polymer literature. Daglen and Tyler showed62 that equation 27 gave excellent fit to these systems as well, which suggests the presence of reaction spheres is common in the mechanism of solid-state polymer photodegradation. 

The majority of drug photostability studies focus on the photodegradation of drugs in solution rather than the solid state. In both cases, photoreactions are based on the absorption of energy in the form of radiation. 

As the molsidomine case shows, the photodegradation of drugs in the solid state can be enhanced by the use of colorants. Another example of this approach are ethinyl estradiol tablets which can be stabilized by the addition of erythrosine (FD C Red No. 3) but degrade faster if they contain l-p-sulfophenylazo-2-naphthol-6 sulfonate, di-sodium salt (FD C Yellow No. 6) (14). 

It is difficult to use reaction order to characterize photodegradation in the solid state. The photochemical processes take place on the product surface, and the change in total concentration measured as a function of irradiation time does not necessarily follow any particular reaction order model. ... 

Photodegradation in the solid state takes place only at the sample surface. The degradation rate is therefore dependent on factors that will influence the depth of light penetration, i.e., change the absorption and reflection at the surface (e.g., particle size, crystal modification, color, thickness of powder bed, and coating of the individual particles or the dosage form). Mefloquine, chloroquine, carbamazepine, and furosemide are examples of drug substances that show different decomposition rates dependent on their polymorphous modiflcation.P ... 

Marciniec, B. and Rychcik, W., 1994, Kinetic analysis of nifedipine photodegradation in the solid state, Pharmazie 49, 894-897. 

The photodegradation pathway of a drug in the solid state does not necessarily follow the degradation in solution. The difference arises mainly from a lack of molecular mobility, which restricts diffusion-controlled reactions and interactions between molecules, thereby influencing the reactant conformation. The photochemical properties of molecules in the solid state or embedded in a solid matrix depend greatly on the organization (if any) and nature of the solid lattice. In a number of cases, it has been possible to correlate the reactivity with packing of the reactant molecule in the crystal (Hadjoudis et al., 1986). 

Marciniec, B., Bugaj, A., and Kedziora, W. (1997) Kinetic studies of the photodegradation of nitroimidazole derivatives in the solid state, Pharmazie, 52, 220-223. 

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