Robert H. Goldsmith St. Mary's College of Maryland St. Mary's City, 20686
Solubility Relationships of Drugs and their Metabolites
Attempts to develop novel experiments in chemistry have been undertaken at this college. Simple model systems were desired that would enable students to compare a drug or common biological material and its metabolite (the material into which the original structure is converted in the bodv). These model svstems should allow us to compare solubility relationshipi of the original material and its metabolite and ~ r e d i c tthe general effect of drug metabolism. The manner in which any chemical is distributed between the -~ two ~ h a s e sof water and the h i d like solvent chloroform sho& how it might be distributed between fattv tissue and water. A comnound's bioloeical activitv is str&ly influenced by how'well it disskves in bbdy tissues which are usually fatty (shown by its fat or chloroform solubility) as well as how quickly the material is removed in the urine (shown by its water solubility). For example, one might expect that highly polar compounds will have poor solubility in the fat or chloroform phase and better solubilitv in the aaueous ~ h a s e It . is desirable to know if a material penetrates these fatty tissues since i t is eenerallv believed that. all else being equal stericallv a material is effect&e io the degree and electrbnica~~y, that it penetrates these tissues.' In the conversion of drug to metabolite, these model experiments should show the general way in which metabolism changes solubility and hence the penetration of the material into lipid tissue. The data should also suggest which material is more water soluble and hence more easily eliminated from the body in the urine. Two model systems that have been developed to illustrate this idea will be presented separately in this paper. A. Salicylic Acid and Gentisic Acid System
Salicylic acid, o-hydroxybenzoic acid, is a well known analgesic and its metabolite is gentisic acid, 2,5-dihydroxybenzoic acid. The metabolic conversion observed here is known as aromatic rine hvdroxvlation. Aaueous svstems of both can be extracted and their aqueous'concen~ations determined bv quantitative colorimetry. Salicylic acid, being a phenol, reacts with ferric ions to give a blue color while gentisic acid also reacts with ferric ions to give a yellowish color due to quinone formation. The only solutions required are a 7.48 x 10W3 M salicylic acid solution, M gentisic acid solution a freshly prepared 7.48 x and e 0.1 M ferric chloride solution which has been stahilized by acid.
Twenty-five milliliters of each acid solution is placed with 25 ml of chlordform in a separatory funnel and shaken vigorously for a 10-min period. The layers are allowed to separate completely and the bottom layer is discarded. With salicylic acid, 5 ml of the aqueous layer is withdrawn and placed in a 50-ml flask that contains 20 ml of distilled water. Five milliliters of the diluted solution is removed and 1 ml of FeCls solution is added to this while shaking gently. After the color develops, the transmittance of the colored solution is read using the Spec 20 at 475 mr. The concentration is determined using the prepared graph. With gentisie acid, 5 ml of the extracted aqueous layer is also withdrawn and placed in the flask but here the dilution involves the addition of 45 ml of distilled water. Five milliliters of this solution is mixed with l ml of FeCl3 solution and the resultant color, is read in the Spec 20 at 595 mr at exactly 1 min after the two solutions were mixed. Taking into account the fivefold dilution of salicylic acid and the tenfold dilution of gentisie acid, the molar concentration of each aeid in its extracted aqueous layer is calculated. Using the aaueaus concentrations and the eiven concentrations of each acid. t h c
