Trimethoprim crystals

R. Beddell, J. N. Champness, D. K. Stammers, and J. Kraut, Refined crystal structure of Escherichia coli and chicken liver dihydrofolate reductase containing bound trimethoprim, J. Biol. Chem. 260 381 (1985). 

E. coli enzyme. But, studies on the chicken liver enzyme may not be relevant to the human enzyme. Indeed, crystal studies of Oefner et al. (1988) on complexes of human DHFR with folate, methotrexate and trimethoprim itself do not support the explanation given by Matthews, and so we are still not sure of the basis for the species selectivity exhibited by trimethoprim. 

The crystal structures of the E. coli DHFR-methotrexate binary complex (Bolin et al., 1982), of the Lactobacillus casei (DHFR-NADPH-methotrexate ternary complex (Filman et al., 1982), of the human DHFR-folate binary complex (Oefner et al., 1988), and of the mouse (DHFR-NADPH-trimethoprim tertiary complex (Stammers et al., 1987) have been resolved at a resolution of 2 A or better. The crystal structures of the mouse DHFR-NADPH-methotrexate (Stammers et al., 1987) and the avian DHFR—phenyltriazine (Volz et al., 1982) complexes were determined at resolutions of 2.5 and 2.9 A, respectively. Recently, the crystal structure of the E. coli DHFR—NADP + binary and DHFR-NADP+-folate tertiary complexes were resolved at resolutions of 2.4 and 2.5 A, respectively (Bystroff et al., 1990). DHFR is therefore the first dehydrogenase system for which so many structures of different complexes have been resolved. Despite less than 30% homology between the amino acid sequences of the E. coli and the L. casei enzymes, the two backbone structures are similar. When the coordinates of 142 a-carbon atoms (out of 159) of E. coli DHFR are matched to equivalent carbons of the L. casei enzyme, the root-mean-square deviation is only 1.07 A (Bolin et al., 1982). Not only are the three-dimensional structures of DHFRs from different sources similar, but, as we shall see later, the overall kinetic schemes for E. coli (Fierke et al., 1987), L. casei (Andrews et al., 1989), and mouse (Thillet et al., 1990) DHFRs have been determined and are also similar. That the structural properties of DHFRs from different sources are very similar, in spite of the considerable differences in their sequences, suggests that in the absence, so far, of structural information for ADHFR it is possible to assume, at least as a first approximation, that the a-carbon chain of the halophilic enzyme will not deviate considerably from those of the nonhalophilic ones. 

The conformations, as determined by crystal structure analyses, of 1-methylweberine, two tetramethoxy weberine analogs, and three polymetho-prims, shown in Fig. 16, are displayed in Figs. 18 and 19. These compounds were synthesized by Takahashi and Brossi (39a) and the crystal structures were determined by J. L. Flippen-Anderson, J. F. Chiang, and I. L. Karle (39b), with the exception of trimethoprim (40) where the three adjacent methoxy groups have the same conformation as shown in Fig. 17(c). 

McDonald, C. Faridah, H. Solubilities of trimethoprim and sulfamethoxazole at various pH values and crystallization of trimethoprim from infusion fluids. J. Parenter. Sci. Technol. 1991, 45, 147-151. 

Cholesterol is reported to crystallize as needles from ethanol and methanol and as platy crystals from acetonitrile. Trimethoprim and sulfamethoxazole have been crystallized in distinctively different habits from different crystallizing solvents (Fig. 2). These studies also revealed that it was possible to obtain different habits of either drug belonging to the same polymorphic state using the same crystallizing solvent by just altering the process variables of crystallization... 

It is important to consider the influence of interaction between functional groups of drugs that leads to their habit modification when formulated in suspension dosage form. Proton transfer from the N atom of sulfamethoxazole to the pyrimidine basic N1 atom of trimethoprim has been reported to occur in their equimolar complexes. Bettinetti et al. have reported nucleation of the complex of trimethoprim and sulfa-methoxypyridazine (1 1) to be accelerated by water or wet granulation. Our studies on cotrimoxazole (unpublished results) revealed immediate formation of fine needle-shaped crystals irrespective of the initial shape of sulfamethoxazole and trimethoprim crystals as a result of the interaction between the two drugs in suspension form. Small needles (Fig. 6A) were... 

Fig. 6. Photomicrographs (magnification 200 x) of crystals showing modified habit produced after interaction between aqueous dispersions of sulfamethoxazole and trimethoprim molar ratio 5.73 1 (A) molar ratio 1 1 (B) HPMC added before mixing aqueous dispersions (C) HPMC added after mixing aqueous dispersions (D).

Bettinetti, G. Caira, M.R. Callegari, A. Merli, M. Sorrenti, M. Tadini, C. Structure and solid-state chemistry of anhydrous and hydrated crystal forms of the trimethoprim-sulphamethoxypyridazine 1 1 molecular complex. J. Pharm. Sci. 2000, 89 (4), 478-489. 

Tiwary AK, Panpalia GM. Influence of crystal habit on trimethoprim suspension formulation. Pharm Res 1999 16 261-265. 

It is a pharmacopoeial requirement that suspensions should be redispersible if they settle on storage. However, the pharmacopoeias do not offer a suitable test that can be used to characterize this aspect of the formulation. In an attempt to remedy this situation, Deicke and Stiverkrtip (1999) have devised a mechanical redispersibility tester, which closely simulates the action of human shaking. The crystal habit may also affect the physical stability of the formulation Tiwary and Panpalia (1999) showed that trimethoprim crystals with the largest aspect ratio showed the best sedimentation volume and redispersibility. 

Another potent inhibitor of DHFR is trimethoprim. The crystal structures of two enzyme-inhibitor complexes (DHFR from Escherichia coli and chicken liver) have been determined [87]. Surprisingly, the inhibitor adopts different conformations in the two proteins. In the chicken liver enzyme, a butterfly-like conformation is observed (t,/t2 = -85°/102 ) in the bacterial enzyme, the aromatic rings are oriented nearly perpendicular to each other ( twisted conformation, r,/T2 = 177°/76°). The inhibitor shows higher affinity for the bacterial enzyme, and the lower energy of the twisted conformation may be partially responsible for this. 

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