Tetracyclines, halogenated

The application of di-arene dihydrodiols has been noted in Part 1 of this chapter, and it is sufficient to note here the application of a mutant of Alcaligenes eutrophus strain B9 that is blocked in the degradation of benzoate (and some halogenated benzoates). This produced the cA-l,2-dihydrodiol (Reiner and Hegeman 1971), and has been used as the source of ring B for the synthesis of a range of tetracyclines (Charest et al. 2005). A range of substituted c -dihydrodiols has been produced, and it has been shown that for 3-substituted benzoates both 3- and 5-substituted cA-dihydrodiols were formed (Reineke et al. 1978). 

Although no new halogen-containing tetracyclines have been reported since the first survey (1), the gene responsible for the chlorination of tetracycline in Streptomyces aureofaciens (Fig. 3.24) has been cloned and the sequence of nucleotides determined (1651). The gene product is a 452 amino acid chlorination enzyme. 

The pH-partition hypothesis relating to membrane penetrability was outlined in Chapter 1. This hypothesis would allow one to make reasonable assumptions of a drug s behavior whose pKa and aqueous-lipid partition were determined. The amphoteric nature of tetracyclines with their multiple pKas, however, presents a more complex situation than simply improving lipid solubility by the deletion of polar hydroxyl groups from positions such as 5 and 6 (e.g., DC, MC, MNC) or the addition of a halogen on C-7 (e.g., DMCTC, CTC). 

Another basic difference between the two materials is that Oasis HLB is macroporous (80 A pores) while LiChrolut EN and Isolut ENV+ are basically nanoporous materials with the maximum pore size distribution located at 20—30 A. For this reason, the latter sorbents perform much better in extracting smaller molecules with molecular masses less than 500 Da, as is the case with the above halogenated acetic acid derivatives [326, 327] or aryl sulfonates [328]. On the contrary, macroporous Oasis HLB is the material of choice when larger analytes are the target of pre-concentration procedures, as was the case with tetracycline and macroHde antibiotics [321-324]. Another vivid example of this kind is pre-concentration of soy isoflavones [331], where Oasis HLB and macroporous Strata X display by far higher recovery values. On the other hand, the size exclusion effect can be exploited purposefully when only smaller molecules have to be retained with the elimination of the major sample matrix. The best example of this type of applications is the analysis of drugs and drug metaboHtes in whole blood or blood plasma [273-275]. Here the microporous hypercrosslinked polystyrene Purosep-270 is the best possible SPE material since it functions as an RAM. 

Tetracycline also can be produced by Streptomyces aureofaciens fermentations under special conditions, that is, chloride starvation or special strains of the organism that fail to halogenate efficiently. Tetracycline possesses many chemotherapeutic properties of chlortetracycline and oxytetracycline. It is an important broad-spectrum antibiotic. 

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