Journal of Pharmacokinetics and Biopharmaceutics, Vol. 1, No. 4, 1973 The Oral Bioavailability and Pharmacokinetics of Soluble and Resin-Bound Forms of Amphetamine and Phentermine in Man Orville N. Hinsvark,1 Aldo P. Truant, 1 Donald J. Jenden, 2 and Joseph A. Steinboru 2 Received Feb. 2, 1973--Final July 31, 1973 Plasma levels of amphetamine and phentermine have been measured in man in a crossover study of the pharmacokinetics of these agents following oral administration of resin-bound and soluble salt formulations. The one-compartment open model with first-order drug absorption was fitted to the data from each subject by nonlinear regression methods and provided an excellent fit. Relative bioavailability of the two salts did not differ for either drug. In both cases the rate constant for absorption was significantly lower and less variable for the resinated compound. KEY WORDS: absorption; amphetamine; bioavailability; elimination; pharmacokinetics; phentermine; resin-bound drugs. INTRODUCTION As one means of controlling their rate of absorption, basic drugs have been incorporated into cation exchange resins (1). The kinetics of release of a drug from the resin particle will determine its concentration in the gastro- intestinal tract and, consequently, its rate of absorption into the circulatory system. Some advantages which may be derived by proper choice of resin and drug are prolongation of action, reduction of peak blood concentrations, and a flatter blood concentration curve. A combination of any or all of these effects may aid in obtaining more predictable responses. To be effective, however, a balance must exist between the drug and the resin matrix: the degree of binding to the resin must be sufficiently strong to accomplish the desired objectives but not strong enough to reduce the bioavailability. The work described illustrates a pharmacokinetic approach to investigate tPharmaceutical Division, Pennwalt Corporation, Rochester, New York 14623. 2Department of Pharmacology, UCLA School of Medicine, Los Angeles, California 90024. 319 (cid:14)9 1973 Plenum Publishing Corporation, 227 West 17th Street, New York, N.Y. 10011.
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`320 Hinsvark, Truant, Jenden, and Steinborn the equivalent bioavailability of resin-bound amphetamine 3 and phenter- mine 4 and their respective soluble salts and to show the effect of resination on the rates of absorption of the individual amines in man. METHODS AND MATERIALS Drug Formulation Amphetamine A combination of one part of d-isomer and one part of dl-racemic mixture was encapsulated in the prescribed amount either of the salt, amphetamine phosphate or of the resinated product, Biphetamine R. Identical capsules were used for the two salts and identified by number only. Phentermine The prescribed amount of this study was encapsulated either as the salt, phentermine hydrochloride, or as its resinated product, Ionamin R. Identical capsules were used for both salts and identified by number only. Drug Administration To minimize individual variations, a crossover design was employed. A minimum of 2 weeks elapsed time was allowed for drug wash-out prior to exposure to the second drug. Regardless of the drug administered, sequential blood samples were drawn into heparinized tubes over the ensuing 24-72 hr, beginning at 30 min. A minimum of 10 samples was drawn in each experiment. The samples were thoroughly mixed and immediately frozen. The actual time and subject were recorded on each tube and all samples were then trans- mitted to the Pharmaceutical Division, Pennwalt Corporation, Rochester, New York, where the drug content in blood was measured. Chemical Measurements The blood samples were maintained frozen until ready for measurement. The drug was extracted into benzene from a measured volume of blood which had been made alkaline with sodium hydroxide. The organic extract was concentrated by evaporation and an accurately measured amount of internal standard was added. The residue was derivatized with trifluoroacetic anhydride, reconstituted to the known volume, and the drug concentration determined by the gas chromatographic method described by O'Brien et al. (2). 3BiphetamineR is the registered trademark of amphetamine bound to the Rohm and Haas IRo120 cation exchange resin. 4IonaminR is the registered trademark of phentermine bound to the Rohm and Haas IR-120 cation exchange resin.
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`Amphetamine and Phentermine in Man 321 Pharmacokinetic Theory Ifa quantity Q0 of drug is administered orally and its absorption follows first-order kinetics, the quantity absorbed (Q) at time t is given by the equation Q = Qo2(1 - e -k't) (1) where 2 is the proportion available for absorption and kl is the first-order rate constant for absorption. Assuming a one-compartment open model, the plasma concentration (Y) is given by Y = [Qo2kl/V(k 1 - k2)](e -kz' - e -k't) (2) where V is the apparent volume of distribution and k 2 is the apparent first- order rate constant for elimination. The assumptions underlying this model have been previously discussed (4,5). Statistical Fitting Procedure Each experiment generated several pairs of values for t and Y : Qo was known and was expressed as mg kg- 1. There are therefore three independent parameters )o be estimated: k 1, k2, and V/2. These parameters were esti- mated by fitting equation (2) to the data from each experiment using a modified Gauss-Newton procedure similar to that described in detail in the BMDX series of biomedical computer programs (3), but employing a Wang 600 electronic desk calculator to carry out the computations. Since the error of estimation is approximately independent of the estimate within the relevant concentration range, the regression procedure employed equal weights for all points. RESULTS The data were found to be satisfactorily fitted by equation (2) for both resinate and soluble salts of amphetamine and phentermine. For every set of data, regression on a function of the form of equation (2) (analysis of variance) was highly significant (P < 0.001 in every case). The standard deviations estimated from the mean residual variances were 2.5 and 2.9 ng/ml for amphetamine and phentermine, respectively. These errors are quantitatively consistent with the reproducibility of the estimation procedure used, and the regression equation is therefore an adequate mathematical model of the kinetic system. Tables I and II summarize the estimates of the kinetic parameters obtained for both salts for amphetamine and phentermine, respectively. Several features of these data deserve comment. Only one of the parameters showed a significant difference between the resinate and soluble salt forms--
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`322 Hinsvark, Truant, Jenden, and Steinborn Table I. Summary of Arithmetic Means _+ Standard Errors of Estimates of Pharmacokinetic Parameters of Amphetamine Phosphate and Resinate in Healthy Subjects Dose Parameters (mg) Salt n k I k 2 V/J. (hr -1) (hr -1) (1 kg -1) 12.5 Phosphate 6 1.514 0.0605 4.23 _+0.439 _+0.0103 _+0.48 Resinate 6 0.651 0.0425 5.06 -+0.144 +0.0111 -+0.36 20 Phosphate 6 1.026 0.0776 3.30 +0.144 _+0.0102 _+0.36 Resinate 6 0.299 0.0958 3.00 -+0.053 _+0.0143 _+0.34 the rate constant for absorption. This was larger for the soluble salt than for the resinate for both drugs at both dose levels, although for the lower dose of amphetamine the difference fell short of significance at the 5 ~ level (0.10 > P > 0.05). For both drugs at both dose levels, this rate constant was also significantly more variable for the soluble salt than for the resinate (variance ratio). None of the other parameters showed significant differences in variability or mean value. For the soluble salts, none of the parameters showed a significant difference between the two dose levels for either drug. For the resinate, the Table II. Summary of Arithmetic Means -+ Standard Errors of Estimate of Pharmacokinetic Parameters of Phentermine Hydro- chloride and Resinate in Healthy Subjects Dose Parameters (mg) Salt n k 1 k 2 V/2 (hr -t) (hr -1) (1 kg -l) 15 Hydrochloride 6 0.614 0.0293 3.30 -+0.100 -+0.0038 _+0.17 Resinate 6 0.295 0.0283 3.30 +0.014 -+0.0024 -+0.17 30 Hydrochloride 8 0.738 0.0316 3.52 ___0.192 +0.0024 -+0.33 Resinate 8 0.236 0.0280 3.81 +0.021 +0.0031 -+0.29
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`Amphetamine and Phentermine in Man 323 rate constant for absorption was for both drugs lower at the higher dose level (0.05 > P > 0.01), while, in the case of amphetamine only, the ratio V/2 was lower (P < 0.01) and the rate constant for elimination was larger (0.02 > P > 0.01) following 20mg than after 12.5mg. The functional importance of these differences is not obvious: one possible explanation is that the pharmacological actions of the drugs may influence their own dis- position. It has been shown that amphetamine delays gastric emptying (6), reduces intestinal motility (7), and delays the absorption of some other agents (8); the prolonged presence of amphetamine or phentermine in the small intestine may delay absorption by causing mucosal vascoconstriction. Figure 1 shows blood concentration curves following hydrochloride and resinate salts of phentermine which are reconstituted from the mean para- meter values presented in Table II, to illustrate graphically the effect of resination. It is evident that the resinate yielded a lower, later, and flatter peak than did the soluble salt, and the tail of the curve was somewhat higher following the resinate. g Z 0 I-- n~ Z 0 U 0 0 3 113 80 60 40 20 I I I RESINATE O ~ 0 20 40 60 80 TIME (hours) Fig. 1. Blood concentrations of phentermine following 0.375 mg kg- 1 of the hydrochloride or resinate salts, calculated from the mean values of kl, k2, and I//2 for 30 mg kg- t summarized in Table II.
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`324 Hinsvark, Truant, Jenden, and Steinborn Since this was a crossover study in which the two salts of each drug were compared in the same subjects at each dose level, it is of interest to analyze the parameter ratios for the two salts. This eliminates intersubject variation and therefore allows a more powerful statistical assessment of the comparative pharmacokinetics of the two salts. In no case did the parameter ratio differ significantly between the higher and lower dose levels, and the data were therefore pooled. The results (Table III) show that of the three parameter ratios studied only kl, the rate constant for absorption, differed significantly from unity. For both drugs, absorption of the soluble salt was approximately three times faster than that of the resinate. It is of particular interest that the ratio of the parameters of V/2 closely approximates unity for both phenter- mine and amphetamine. Since the apparent volume of distribution V should be consistent within a given subject, this implies that the bioavailability 2 is the same for resinate and soluble salts. DISCUSSION A simple variant of the one-compartment open model has been used to compare in man the pharmacokinetics of soluble salts and insoluble resinates of phentermine and amphetamine. The model has been used extensively in the literature, and its use and limitations are considered in detail in a recent comprehensive work (4). Wagner et al. have adapted this and related models to the estimation of bioavailability (4 : Chapter 39; 5). The theoretical basis of the present study is essentially the same as that used by Wagner et al. (9), to compare the bioavailability of spectinomycin follow- ing intramuscular and intravenous injection in the same subjects. Although the data presented here do not allow an absolute assessment of the efficiency of oral absorption of amphetamine and phentermine, the estimation of relative absorption of resinates and soluble salts rests upon a similar basis. Table IlL Summary of Mean Parameter Ratios for Soluble/Resinate Salts in the Same Subjects ~ Parameter Amphetamine (n = 12) Phentermine (n = 14) ratio Mean ratio Mean ratio +SE t P +SE t P k~ 3.356 + 0.756 4.439 <0.001 2.814 _ 0.450 4.028 <0.01 k 2 1.597 _+ 0.507 1.177 >0.2 t.147 + 0.098 1.500 >0.1 V/2 0.991 + 0.083 -0.110 >0.2 0.985 __+ 0.072 -0.206 >0.2 aThey are assessed in relation to the null hypothesis that the expected ratio is unity. None of the assessments was altered by analyzing the reciprocal ratios or the logarithms of the ratios or the arithmetic differences in the parameter estimates.
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`Amphetamine and Phentermine in Man 325 Procedures have long been known for least-squares determination of exponential or other nonlinear parameters (1031) but they require extensive computation and until recently they have not been practical to implement. Contemporary electronic computers and sophisticated computational techniques have made nonlinear parameter determination readily accessible. In particular, the Gauss-Newton method of determining nonlinear coeffi- cients has been used extensively (11-13). The use of this method for deter- mination of the exponential parameters provides least-squares estimates of the parameter values and asymptotic estimates of their standard errors. Although nonlinear regression yields estimates which may be biased (14), the bias is about an order of magnitude less than the standard error of the estimates and is negligible in relation to it (14,15). Discussions of the statistical properties of the algorithm and of the computational procedures are found elsewhere (16,17) and will not be discussed here. Computer programs are also readily available and their usage well documented. For example, a description of a Fortran program to determine the Gauss-Newton non- linear least-squares estimates is available in the BMDX series of the bio- medical computer programs (3). The results in this paper, however, were obtained by use of the Gauss- Newton method programmed for the Wang 600 electronic desk calculator. Once a data set is keyed into the calculator, the solution can be obtained in about 30 sec to 1 min depending on the observational error in the data. Curves with about 45 data points can be processed with editing and other clerical necessities at about four per hour by use of this procedure, including computation of the theoretical curve for plotting purposes. The basic conclusions to be drawn from the results appear fairly clear- cut. Efficiency of absorption of amphetamine and phentermine after oral administration was the same for resinates as for soluble salts, but the speed of absorption was on the average about three times slower for the resinate. Since the other pharmacokinetic parameters were the same for both salts, blood levels after the resinate reached a lower, later, and flatter peak, which is illustrated in Fig. 1. This may be relevant to the avoidance of the euphoriant effect of amphetamine, which is closely correlated with peak plasma levels after intravenous injection in man (18). The rate constant for absorption in all groups had a significantly larger variance after the soluble salts than after the resinates in the same subjects; the rate of absorption of the resinate is therefore more consistent than that of the hydrochloride or phosphate, and is likely to result in more consistent as well as more sus- tained blood levels. The apparent volume of distribution was similar for amphetamine and phentermine and exceeded total body volume by a factor of 3 to 5. Similar results have recently been reported in a study of the pharmacokinetics of
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`326 Hinsvark, Truant, Jenden, and Steinborn amphetamine and phentermine in rats (19). The clearance and rate constant for elimination of amphetamine exceeded those of phentermine by a factor of 2 to 3, indicating a longer duration of action for the latter agent. This may reflect a greater resistance of phentermine to metabolic attack ; little is known of the metabolic fate of this drug (20), although amphetamine has been extensively studied both in experimental animals and in man (21,22). The elimination constant of amphetamine is lower than in a previous study (18), in which the urine was acidified by ammonium chloride in order to accelerate excretion (23). Bioavailability is commonly estimated in crossover studies from the ratio of the integrals of the plasma concentration curves as a function of time (24). This method is correct if elimination follows first-order kinetics and the rate constant is the same for both halves of the crossover experiment. Under these conditions integration will lead to conclusions similar to those 60- 50 4O uJ 5O I-- I I I 20 io -o 2o 4o 6o eo BLOOD CONCENTRATION (ng ml -~) Fig. 2. Bioavailability of phentermine (circles) and amphetamine (squares) as resinates (open symbols) and soluble salts (filled symbols) expressed as the average time interval (ordinate) following a single dose for which the blood concentration exceeded a given threshold (abscissa).
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`Amphetamine and Phentermine in Man 327 yielded by the method used here provided it is unbiased and the integral beyond the last observation point can be accurately estimated. A correction has been Suggested if the rate constant changes between treatments (4). Use of maximum likelihood estimates of the parameter ratio V/2 avoids these difficulties with little loss in generality. In the experiments presented here, estimation of the area under the curve would be subject to considerable error because 10-25 ~ of the area lies beyond the last observation points and can only be estimated crudely by extrapolation. Figure 2 presents an empirical bioavailability comparison of resinate and soluble salts, in which threshold blood curve is plotted against the average length of time for which the threshold was exceeded. It is clear that there is little difference between the two salts for blood levels less than 40 and 60 ng ml-1 for amphetamine and phentermine, respectively; blood levels of 50 and 75 ng ml- 1 or higher were not achieved after administration of the resinates. This analysis is independent of the pharmacokinetic model used, and it confirms the conclusion that one of the chief effects of resination is to reduce the peak blood concentration achieved following an oral dose without otherwise changing the bioavailability to a significant degree. ACKNOWLEDGMENTS The authors wish to acknowledge the assistance given to them by the laboratory personnel, V. Abbey, G. Hanley, W. Kuipers, J. O'Brien, A. Piccirilli, and W. Zazulak who collected the data ; to C. Leuwen for preparing the manuscript ; to the clinical investigators, Dr. Jolly of Biometric Testing, Inc., Englewood Cliffs, N.J. and Dr. Zone of Rochester General Hospital, Rochester, N.Y., and to Dr. Cohn of Peninsular Testing Corp., Miami, Fla., for the administration of the drugs and for the collection of samples. REFERENCES 1. C. Calmon and T. R. E Kressman. Ion Exchanges in Organic and Biochemistry. Interscience Publishers, Inc., New York, 1957, pp. 481-483. 2. J. E. O'Brien, W. Zazulak, V. Abbey, and O. Hinsvark. Determination of amphetamine and phentermine in biological fluids. J. Chrom. Sci. 10:336-341 (1972). 3. W. J. Dixon. BMD Biomedical Computer Programs. University of California Press, Berkeley, California, 1968. 4. J. G. Wagner. Biopharmaceutics and Relevant Pharmacokinetics. Drug Intelligence Publica- tions, Hamilton, I11., 1971, 5. J. G. Wagner. Method of estimating relative absorption of a drug in a series of clinical studies in which blood levels are measured after single and/or multiple doses. J. Pharm. Sci. 56:652-653 (1967). 6. D. W. Northup and E. J. Van Liere. Effect of the isomers of amphetamine and desoxy- ephedrine of gastric emptying in man. J. Pharmacol. 109:358-360 (1953).
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`328 Hinsvark, Truant, Jenden, and Steinborn 7. E. J. Van Liere, J. C. Stickney, D. W. Northup, and R. O. Bell. Effect of dl-amphetamine sulfate and its isomers on intestinal motility. J. Pharmacol. 103:187-189 (1951). 8. H. H. Frey and E. Kampmann. Interaction of amphetamine with anticonvulsant drugs. II. Effect of amphetamine on the absorption of anticonvulsant drugs. A cta Pharmacol. Toxicol. 24:310-316 (1966). 9. J. G. Wagner, E. Novak, L. G. Leslie, and C. M. Metzler. Absorption distribution and elimination of spectinomycin dihydrochloride in man. Intern. J. Clin. Pharmacol. 1: 261-285 (1968). 10. C. F. Gauss. Theoria motus corporum coelestium. Werke 7:240-254 (1809). 11. N. R. Draper and H. Smith. Applied Regression Analysis, John Wiley & Sons, New York, 1966. 12. G. E. P. Box. Fitting empirical data. Ann. N. Y. Acad. Sci. Art. 3 86:792-816 (1960). 13. H. O. Hartley. The modified Gauss-Newton method for the fitting of nonlinear regression functions by least squares. Technometrics 3:269-280 (1961). 14. M. J. Box. Bias in non-linear estimation. J. Am. Star. Soc. (B) 33:171-190 (1971). 15. R. R. Kinnison. Statistical characteristics of estimates from a non-linear least squares problem in drug distribution. Ph.D. dissertation, UCLA, Los Angeles, California, 1971. 16. R. I. Jennrich and P. F. Sampson. Application of stepwise regression in non-linear estima- tion. Technometrics 10:63-72 (1968). 17. R. I. Jennrich. Asymptotic properties of non-linear least squares estimators. Ann. Math. Stat. 40:633-643 (1969). 18. L. E. J6nsson, E. NnggSrd, and L. M. Gunne. Blockade of intravenous amphetamine euphoria in man. Clin. Pharmacol. Therap. 12:889-896 (1972). 19. A. K. Cho, B. J. Hodshon, B. Lindeke, and G. Miwa. Application of quantitative gas chromatography/mass spectrometry (GC/MS) to a study of the pharmacokinetics of amphetamine and phentermine. J. Pharm. Sci. 62:1491-1494 (1973). 20. A. H. Beckett and L. G. Brookes. The metabolism and urinary excretion in man ofphenter- mine and the influence of N-methyl and p-chloro substitution. J. Pharm. Pharmacol. 23" 288-294 (1971). 21. R. L. Smith and L. G. Dring. Patterns of metabolism of/~-phenyl-isopropylamines in man and other species. In Amphetamine and Related Compounds: Proceedings of the Mario Negri Institute for Pharmacological Research, E. Costa and S. Garrattini, eds., Raven Press, N.Y., 1970, pp. 121-139. 22. A. H. Beckett and S. A1-Sarraj. The mechanism of oxidation of amphetamine enantio- morphs by liver microsomal preparations from different species. J. Pharm. Pharmacol. 24:174-176 (1972). 23. J. M. Davis. I. J. Kopin, L. Lemberger, and J. Axelrod. Effects of urinarypH on ampheta- mine metabolism. In Drug Metabolism in Man, E. S. Vessell, ed., Ann. N.Y. Acad. Sci. 179:493-501 (1971). 24. J. G. Wagner. An overview of the analysis and interpretation of bioavailability studies in man. Pharmacology 8:102-117 (1972).
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