Cho M J, Kurtz R R, Lewis C, Machkovech S M, Houser D J
J Pharm Sci. 1982 Apr;71(4):410-4. doi: 10.1002/jps.2600710409.
To develop a parenteral solution of relatively water-insoluble metronidazole (2-methyl-5-nitro-1H-imidazole-1-ethanol), its phosphate ester was synthesized via two routes. One route utilized 2-cyanoethyl phosphate and the other utilized pyrophosphoryl tetrachloride. The first method used dicyclohexylcarbodiimide as a coupling agent and the cyanoethyl group was removed under mild alkaline conditions. The second method was a one-step procedure in which free acid of metronidazole phosphate was isolated as a crystalline solid. The solubility of metronidazole in various solvents was determined at 25 degrees. From the pH-dependence of its aqueous solubility, the pKa of the conjugate acid of metronidazole was estimated to be 2.62, which agreed well with the pKa values of other nitroimidazoles. Metronidazole phosphate behaved as a zwitterionic compound in an acidic medium with a minimum solubility at pH 2.0. At pH 7, its solubility was approximately 50 times that of metronidazole. The phosphate ester was so soluble at pH higher than 7 that it was difficult to measure the solubility accurately. In human serum, the hydrolysis of metronidazole phosphate followed zero-order kinetics at an initial concentration of 0.25 mg/ml or higher, presumably due to enzyme saturation (0.035 mg/ml/hr at 37 degrees). A reversed-phase HPLC procedure was adopted to monitor the appearance of metronidazole and the disappearance of metronidazole phosphate. Subcutaneous administration of metronidazole phosphate to rats produced a blood level of bioactivity comparable to that observed after administration of metronidazole.
为研发相对水不溶性甲硝唑（2-甲基-5-硝基-1H-咪唑-1-乙醇）的肠胃外溶液，通过两条路线合成了其磷酸酯。一条路线使用2-氰基乙基磷酸酯，另一条路线使用焦磷酰四氯。第一种方法使用二环己基碳二亚胺作为偶联剂，氰基乙基在温和碱性条件下被除去。第二种方法是一步法，其中磷酸甲硝唑的游离酸被分离为结晶固体。在25℃下测定了甲硝唑在各种溶剂中的溶解度。根据其在水中溶解度的pH依赖性，估计甲硝唑共轭酸的pKa为2.62，这与其他硝基咪唑的pKa值吻合良好。磷酸甲硝唑在酸性介质中表现为两性离子化合物，在pH 2.0时溶解度最小。在pH 7时，其溶解度约为甲硝唑的50倍。磷酸酯在pH高于7时非常易溶，以至于难以准确测量其溶解度。在人血清中，磷酸甲硝唑的水解在初始浓度为0.25mg/ml或更高时遵循零级动力学，这可能是由于酶饱和（37℃时为0.035mg/ml/小时）。采用反相高效液相色谱法监测甲硝唑的出现和磷酸甲硝唑的消失。对大鼠皮下注射磷酸甲硝唑产生的生物活性血药浓度与注射甲硝唑后观察到的相当。
