Spectrophotometric assay of levodopa and alpha-methyldopa in pharmaceutical products and in biological samples of schizophrenic patients using copper complex and tri-iodide reagent

ABSTRACT

Two simple, rapid and sensitive spectrophotometric methods are proposed for the determination of levodopa [LD] I, and alpha-methyldopa [alpha -MD] II. The first method is based on coupling of 4-aminoantipyrine [4-AAP] with LD and a alpha -MD to give new ligands that react with copper-tetramine complex to give intense colored chelates of ratios [111, LD or alpha -MD 4-AAP copper tetramine]. The colored products are quantified spectrophotometrically at 520 and 545 nm for LD and a alpha -MD, respectively. The optimization of the experimental conditions is described. The method has been used for the determination of 19.72-69.02 and 31.24-98.94 micro g ml[-1] of drugs I and II, respectively. The accuracy of the method is indicated by the values of recovery [100 +/- 0.2%] and the precision is supported by the low standard deviation [SD 0.17-0.22] and relative standard deviation [RSD=0.59-1.54%]. The second method is based on the formation of ion-pair iodinated inner sphere or outer sphere colored complexes between the drug and iodine or tri-iodide negative ions at pH 5 and room temperature [25 +/- 3°C]. This method has been used for the determination of LD within the concentration range 39.44-78.88 micro g ml[-1] with SD=022-0.24 and recovery percent=100 +/- 0.3%. The sensitivity of the two methods is indicated by Sandell factor of 0.014- 0.016 g cm[2]. The results of the two methods are compared with those of the official method. The interference from common drug additives, degradation products and excepients was also studied. The proposed methods were applied successfully to the determination of LD and alpha-MD in dosage forms. The second method was also successfully applied for the analysis of LD and carbidopa [CD] synthetic mixture of concentration of these drugs similar to that of Aldomost drug form of LD, and also the determination of these drugs in urine of some schizophrenic patients with good precision and accuracy. The reliability of the methods was established by parallel determinations against the official British pharmacopoeia method: