different commercial firms. The materials bought were: I,

Transcription

1 TI-IIOPENTAL LEVELS IN CANINE PLASMA AND URINE DURING THE FIRST HALF-HOUR POST-INJECTION PERIOD AS DETERMINED BY LIQUID SCINTILLA- TION COUNTING AND GAS CHROMATOGRAPHY* LeRoy I. Braddock and Henry L. Price Department of Pediatrics, Hahnemann Medical College, Philadelphia, Pa., and Department of Anesthesia, School of Medicine, University of Pennsylvania, Philadelphia, Pa. Introduction Three different samples of radioactive thiopental, two containing S-35 and one a C-l4, were assayed for chemical purity; and the "in vivo" effect of the purity was studied in dogs. The purpose was to evaluate the feasibility of using liquid scintillation counting and gas liquid chromatography in studies on the distribution and metabolism of thiopental during the period of maximum anesthetic effect. The data tend to support the feasibility of such use. Experimental Radioactive tagged samples of 5-ethyl-5 (1-methylbutyl)- 2-thiobarbituric acid (thiopental) were purchased from three different commercial firms. The materials bought were: I, thiopental-s35-sodium; II, thiopental-s35-acid; and III, thicpental- *Tffls research was supported in part by the National Institutes of Health, United States Public Health Service, Bethesda, Md. under grant GM Acknowledgement is also given for the aid of Dr. G. Robinson and Dr. M. Heddin. 599

2 LEROY I. BRADDOCK AND henry L. PRICE 2-C14-sodium. The specific activity of each material was about 40 uc/mg. The liquid scintillation solution (EDAM) was prepared as recommended by A. Eisenhardt (1). Spectral grade dioxane and dimethylparaphenylene -bi s-5 -phenyloxazole (DMFOPOP) were mixed in small portions with stirring until 150 mg of DMFOPOP were completely dissolved in 600 ml. of the dioxane. The stirring was continued while g. of 2, 5-diphenyloxazole (PPO) were dissolved. With continued stirring, the following were added in the order listed: 100 ml. of anisole, 100 ml. of 1,2- dimethoxyethane, and 200 ml. of ethylene glycol. The final volume was about one liter. The solution was stored in a dark bottle, tightly closed and refrigerated. It was miscible with both aqueous solutions, organic solvents and most physiological systems encountered in clinical research. Counting was done with an automatic tri-carb liquid scintillation counter following common procedures (2). Gas chromatographic data were obtained with an F&M Model 400 flame ionization instrument and a 3. 8% SE-30 packed column as described elsewhere (3). Because of a demonstrated tendency for thiopental to he desulfurized during "in vitro" handling and evaporations, gas chromatographic results reported as thiopental are the summation of the observed thiopental and pentobarbital (its oxygen analog) responses adjusted for the molecular weight differences. All data, when appropriate, were corrected for recovery losses during analysis. The mean recovery, as estimated from liquid scintillation counting, was about 90%. The "in vivo" experiments were run using mongrel bitches weighing between 10 and 15 kg. The animals were initially anesthetized with either halothane or nitrous oxide, fitted with an endotrachial tube, and connected to a positive pressure respiratory unit. Catheters were inserted in a femoral artery and in the urethra. An intravenous drip of 5% dextrose solution was maintained at a rate sufficient to yield an adequate urinary flow. Blood drawn was replaced with normal saline. After drawing a preliminary blood sample and emptying the bladder, the anesthetic gas was discontinued. Just prior to awakening the animal received 200 mg. i. v. of buffered thiopental sodium solution (Abbot) which had been adulterated with 0.02% by weight of 600

3 ORGANIC SCINTILLATORS radioactive material. A suitable level of anesthesia was maintained with halothane or nitrous oxide. The red cells were centrifuged upon collection. The urine collections were examined for blood by benzidine test and rejected if positive. Chemical assay of radioactive material was done as follows: A standard aqueous sample of thiopental was prepared in an alkaline medium with a ph from 10 to 12. Aliquots were acidified with HC1 and thrice extracted with chloroform (2. 5 parts organic to 1 aqueous). Each aqueous residue was made up to standard volume. The chloroform extracts were combined, evaporated to dryness and redissolved in a known amount of benzene. The benzene solutions were examined by gas chromatography. Radioactive counts were taken on the original standards, the aqueous extraction residues, and the benzene solutions. Spectrophotometric measurements on thiopental sodium dissolved in ph 11 buffer were taken at 245 mu and 300 mu and compared with standard curves for thiopental and pentobarbital. Radioactive assays were also made and specific activities were found to be in agreement with the specific values. The observed results are presented in the tables. Results and Discussion Radioactive material I had about half the S-35 tag on the thiopental. The barbiturate level was about a third pentobarbital, as shown by both gas chromatography and spectrophotometry. The other two thiopental preparations were reasonably pure. The pentobarbital content was only a few percent in each, and the tag seemed to have been on the thiopental molecule. Sin the fraction of tagged thiopental in the i.v. injection solutions was so low, no error would be expected from the small amount of pentobarbital introduced. However, in the case of material I, where about half the radioactive tag was not on the barbiturate, a misinterpretation could easily occur. The "in vivo" results indicate the amount of radioactivity in the urine, during the first half-hour post-injection period, to have been about ten-fold greater than when the other two compounds were used. Materials II and III yielded comparable data. Such data only reiterates the usual warning for one to be certain 601

4 LEROY I BRADDOCK AND HENRY L PRICE of the position of the tagged atom. Specific activity assays, alone, are insufficient. Insystems where the presence of degradation products would be minor, such as the plasma during the time period studied, one would expect agreement between liquid scintillation counting results and those from gas chromatography. This was true for materials II and III. On the other hand, this situation does not prevail in the urine, and the data obtained indicated only about 10-20% of the excreted radioactivity to be accounted for by thiopental excretion. Also, this level tended towards lower values as the post-injection time became longer. Whether or not drug metabolism plays a prominent role during the first half-hour, as suggested recently by some authors (4, 5), cannot be verified by urinary excretion data alone. Metabolites are not necessarily excreted upon formation. However, the amounts of radioactivity excreted during the period of maximum anesthetic effect were quite low in this study, generally being less than 0. 5% of the total dose when the tag was attached to the barbiturate molecule. On the other hand, very small amounts of detoxified material did appear in the urine within the 5-15 minute post-injection period. One can conclude, from the data presented, that both liquid scintillation counting and gas liquid chromatography have adequate sensitivity for distribution and metabolic studies of the type discussed. The purpose of the study will be the deciding factor. TI one is concerned oniy with thiopental levels, radioactivity counts will give high results. Conversely, gas chromatography will provide low estimates if detoxification products are present. Both techniques are more sensitive than the classical spectrophotometric methods, and are comparable in sensitivity to fluorometry (6). It is evident that the two techniques, liquid scintillation counting and gas chromatography, complement each other. References 1. A. Eisenhardt, Private communication, February, 1965; School of Medicine, University of Penna, Philadelphia, Pa. 602

5 ORGANIC SCINTILLATORS L. I. Braddock and N. Marec, J. Gas Chrom. 3, 274 (1965). G. D. Chase and J. L. Rabinowitz, "Principles of Radioisotope Methodology", p , Burgess Publishing Co., Minneapolis, Minn., L. J. Saidman and E. I. Eger III, Anesth. 27, 118 (1966). K. B. Bischoff and R. L. Dedrick, j. Pharm. Sci. 57, 1346 (1968). P. G. Dayton et al, Biochem. Pharmacol. 16, 2321 (1967). TABLE 1 Technique Weight Percent of Commercial Radioactive Materials Actually Present as Barbiturate i ii liii Thiopental Thiopental Thiopental S35-Sodium S35-Acid C14-Sodium Liq. Scint. Ct Gas Liq. Chrom UV Spectr TABLE 2 Canine Urinary Excretion as Accumulative Percent Dose Liquid Scintillation Counting and I Thiopental-S35-Sodllum, Purity 54%, 200 mg. Dose Animal 1 Animal 2 Animal 3 Time Time % Time 0.5 hr mm. None 5 mm. None

6 LEROY I. BRADDOCK AND HENRY L. PRICE TABLE 3 Canine Urine Levels During Thiopental Anesthesia. Comparison of Liquid Scintillation Counting (LSC) and Gas Liquid Chromatography (GLC). 200 mg. Dose Thiopental-S35-Acid, Animal 4 ug. /cc. Accumulative Time Volume LSC GLC o/o Dose 2 mill. 25 cc. 0.1 None None Thiopental-.Cl4-Sodium, Animal 5 ug. /cc. Accumulative Time Volume LSC GLC o/o Dose 5 mm. 50 cc. None Thiopental-C14-Sodium, Animal 6 ug. /cc. Accumulative Time Volume LSC GLC o/o Dose 25 mm. 10cc

7 ORGANIC SCINTILLATORS TABLE 4 Canine Plasma Levels During Thiopental Anesthesia Comparison of Liquid Scintillation Counting (LSC) and Gas Liquid Chromatography (GLC). 200 mg. Dose Thiopental-S35-Acid Thiopental-C 14-Sodium Animal 4 Animal 6 ug/cc ug/cc Time LSC GLC Time LSC GLC 2 mm mm