Pharmacokinetics of phenylpropanolamine in humans after a single-dose study

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1 International Jornal of Pharmacetics, 39 (1987) Elsevier 141 I1P Pharmacokinetics of phenylpropanolamine in hmans after a single-dose stdy R. Dose, J.M. Haigh and I. Kanfer School of Pharmacetical Sciences, Rhodes University, Grahamston (Soth Africa) (Received 8 November 1986) (Modified version received 18 April 1987) (Accepted 15 March 1987) Smmary Key ords: Phenylpropanolamine; Serm; Urine; Bioavailability; Pharmacokinetics; Modelling The pharmacokinetics of phenylpropanolamine have been stdied in healthy hman volnteers folloing the oral administration of an aqeos soltion of the drg (50 mg/200 rnl). Blood and rine samples collected throghot the trial ere assayed sing HPLC ith UV detection. The drg as shon to be rapidly absorbed ith a mean tmax of 1.47:!: 0.49 h and a mean elimination half-life of 4.0:!: 0.5 h. Phenylpropanolamine is predominantly excreted via the kidney ith a mean renal clearance of 0.646:!: liter/kg/h and 90.2:!: 1.7% excreted nchanged in the rine. The data ere not ell described sing conventional one or to body compartment models. Hoever, the incorporation of a discontinos absorption phase into the models reslted in an improved overall fit ith better characterisation of the absorption phase. Introdction Phenylpropanolamine (PPA), a sympathomimetic amine, is idely sed as a nasal decongestant and is a component of many proprietary cold preparations. In larger doses, PPA is sed as an appetite sppressant and is one of the most poplar over-the-conter appetite sppressants in the U.S.A. Hoever, over the past fe years, PPA has been the focs of mch controversy de to its reported ndesirable side effects and the potential of this drg for abse. Many reports and letters have appeared in the literatre condemning the se of PPA (King, 1979; Horoit,et al., 1980; Correspondence: I. Kanfer, School of Pharmacetical Sciences, Rhodes University, Grahamston 6140, Soth Africa. Bernstein and Diskant, 1982; Greenood, 1983; McDoell and LeBlanc, 1985). Despite its extensive se, a literatre srvey revealed a distinct pacity of information concerning the kinetics of absorption and disposition of PPA (Dose, 1984). The absence of sch pharmacokinetic data can be attribted to the lack of an analytical method sensitive enogh to determine the lo concentrations of PPA fond in plasma. Previos kinetic stdies ere limited to the determination of PPA in rine only (Heimlich et al., 1961; Beckett and Wilkinson, 1965; Wilkinson and Beckett, 1968a and b) and the only pharmacokinetic parameters, half-life and absorption rate reported to date, have been based on these rine data. A pharmacokinetic stdy of PPA sing both blood and rine data described the steady-state 1987 Elsevier Science Pblishers B.V. (Biomedical Division)

2 142 kinetics of the drg administered in a sstained-release formlation (Lonnerholm et ai., 1984). The apparent mean half-life as fond to be 5.6 h. Vales ere also calclated for mean steady-state concentration, area nder the plasma concentration-time crve, apparent volme of distribtion and renal clearance. The present stdy as ndertaken to examine the absorption and disposition of PPA in hmans folloing the administration of an aqeos soltion of the drg. A nely developed, sensitive, specific HPLC assay (Dose et ai., 1983) as sed to determine serm. and rine concentrations of the drg. Materials and Methods Trial design Six healthy volnteers ho ere non-smokers participated in the trial. They ranged in age from 21 to 28 years and eighed beteen 54 and 79 kg. Volnteers ere chosen on the basis of intervie, physical health and standard laboratory tests hich inclded haematology, blood chemistry and rinalysis. Informed consent as obtained from each sbject. Folloing an overnight fast, each sbject received a test dose. A standardied breakfast as served 2 h after the start of the trial. Trial schedle: 50 mg PPA hydrocworide (PPA.HCI) dissolved in 200 ml of ater as administered to each of 6 sbjects. Blood samples ere dran before dosing and at 10-min intervals for the first h, then at 1, 1i, 2, 3, 4, 6, 8, 10, 12, 14, 16, 18 and 24 h after administration of the drg. Serm as separated by centrifgation and rine samples ere collected throghot the trial. All samples ere stored at C ntil analysis. Analytical method Serm and rine concentrations of PPA.HCI ere determined by HPLC sing direct UV detection. The method involved chloroform extraction at a basic ph and a single back-extraction into 5% viv acetic acid (Dose et ai., 1983). An aliqot of the acidic extract as injected directly onto the colmn. The limit of detection as 25 ng/ml at hich the coefficient of variation as 5.2%. Calclation of bioavailability parameters Maximm PPA serm concentrations, Cmax,and the time to obtain this concentration, tmax' ere determined from individal PPA serm concentration profiles. Area nder the crve (AUC) data ere calclated sing the linear trapeoidal eqation in the absorption phase and the log trapeoidal method in the postabsorption phase here the fnction declines monoexponentially. The terminal rate constant, Kel, as calclated from the terminal slope of a semilog plot of serm concentration vs time and the elimination half-life, t~, as ths obtained. The apparent volme of distribtion, V, as estimated sing the folloing eqation, assming F=l: F.D V= AUCoo.KeI (1) here D is the dose in ILg. The percent of the dose excreted nchanged, %E, as determined directly from rine data. The excretion rates ere plotted against the serm concentrations at the mid-points of the excretion intervals and renal clearance as obtained from the slope of this line. Modelling of data Individal serm concentration-time data ere fitted by non-linear regression analysis to varios models (see Fig. 1) sing the program NONLIN (Metler, 1974) ith eqal eighting to the data points. Eqn. 2 describes a one-body compartment model (lbcm) ith first order absorption and Eqn. 3 characterises a 2BCM ith first order absorption and elimination solely from the central compartment. dc dt = k D e-kap-io) - KlOCl ax V (2) dcl k D -ka (1-10) + K A2 - K 12 C - dt = ax-v e x 21 V l - KlOCl The interpretation (3) of PPA absorption as ero order

3 143 MODEL A A1 k1 ) A2~~ A3~~~ A4 MODEL B ko B1 k1 ) B2~~ B3~~~ COMPARTMENT ONE lklo COMPARTMENT ONE 1 klo ~ ~ Fig. 1. Pharmacokinetic models for phenylpropanolamine. COMPARTMENT inpt as assessed by fitting the data to the folloing eqation: c = ko(ekiot-1) e-kioi VKlO TWO (4) here dc/dt = rate of change of drg concentration in the central compartment kax D V A2 C1 KlO K12 K21 t to = first, second or third absorption rate constant = dose of the drg = volme of the central compartment = amont of drg in the peripheral compartment = concentration ment in the central compart- = elimination rate constant = first order rate constant for transfer of drg from the central to the peripheral compartment = first order rate constant for transfer of drg from the peripheral to the central compartment = time after drg administration = lag time beteen drg administration and appearance of drg in the serm ko = apparent constant ero order absorption rate T = a constant corresponding to the absorption time, after absorption apparently ceases. (Dring absorption, T is a variable and eqal to t) The possibility of discontinos absorption of PPA as investigated by modifying the differential eqations describing the model to inclde 2 or 3 absorption rate constants (see Fig. 1). The times at hich the absorption rates changed ere estimated by stdying the individal serm concentration-time profiles. Goodness of fit as assessed by calclating the correlation coefficient for each set of observed and predicted data, and by visal observation of the crves. Akaike's Information Criterion (AIC) as sed to select an appropriate model describing the experimental data (Yamaoka et al., 1978), ith the eqation having the minimm AIC being regarded as best characterising the data set. Ale. = nlnre. + 2p J J (5) Where additional parameters ere added to a model, the F ratio test (Boxenbam et al., 1974) as applied to determine hether or not the eighted sms of sqared deviations ere sfficiently redced to jstify fitting ith the additional parameters, Rej - Rek nk - Pk F= ( Rek ) x ( (nj-p)-(nk-pk) ) (6) here n is the nmber of observations, P, the nmber of parameters, Re, the eighted sm of sqared deviations and j and k represent the jlh or kthdata set. Reslts Pharmacokinetic data Table 1 smmarises the pharmacokinetic data. Mean serm and rine profiles (:t S.D.) are depicted in Figs. 2 and 3, respectively. Absorption of

4 144 TABLE 1 Pharmacokinetic parameters for PPA Sbject Weight tmax Cmax AUCoo Kel t! 2 %E V CIR (kg) (h) (ngjml) (ngjml. h) (h-i) (h) (liter jkg) (liter jkgjh) Mean IS.D %E is the percentage of nchanged drg excreted in the rine; Cl R is renal clearance. "..J oo >::, '-' a: >:: => 25 en 0 -I' 0 TIME 6 B [HOURS) Fig. 2. Mean PPA serm concentrations (IS.D.) folloing the oral administration of a 50 mg aqeos soltion of the drg. '-' >:: ~ 20 >< ::;: 10 g >:: a: B TIME (HOURS) Fig. 3. Mean PPA rine excretion crve (I S.D.) folloing the oral administration of a 50 mg aqeos soltion of the drg. 14 the drg is rapid, ith peak concentrations occrring beteen 50min and 2 h after ingestion, hich agrees ith the reslts of a previos stdy (Mason and Amick, 1981). Peak concentrations range from 112 ng/ml to 186 ng/mi. The apparent volme of distribtion is large, 4.5 i: 0.7 liter/kg, hich old accont for the lo concentrations of PPA fond in the serm. This rather high distribtion volme indicated that PPA is extensively bond to extravasclar sites. The tisse distribtion in dogs 2 h after administration as fond to be kidney> lng> liver> spleen> brain> heart> mscle> plasma> fat> CSF (Axelrod, 1953). Half-life vales ere fond to range beteen 3.0 and 4.4 h, similar to vales calclated in previos rinary excretion stdies (Heimlich et al., 1961; Wilkinson and Beckett, 1968a). PPA is reported to be completely absorbed, ith approximately 90% of the nchanged drg being excreted in the rine in 24 h (Heimlich et al., 1961). In one stdy (Sinsheimer et ai., 1973) only 4% as fond as transformation prodcts. The percent of nchanged PPA recovered from the rine ranged from 89 to 93% (Table 1), spporting the theory of complete absorption and negligible metabolism of the drg. The relationship beteen renal clearance and serm concentration as determined nder dynamic conditions, as serm concentrations of PPA ere not at steady-state and changed rapidly. Large intra-individal differences beteen the

5 145 sbjects in renal clearance occrred dring the trials. The mean renal clearance of literjkgjh exceeds the glomerlar filtration rate, indicating that a sbstantial proportion of the dose as eliminated by tblar secretion. Flo rate and rine ph have been shon to have an effect on the clearance of certain drgs (Roland and Toer, 1980). Urine ph as monitored throghot the trial and as fond to flctate beteen 5.5 and 7.5. Phenylpropanolamine, being a eak base ith a pka of 9.4 (Kanfer et al., 1983), old not ndergo significant tblar reabsorption at these ph vales. Modelling The mean pharmacokinetic parameters from fitting the serm data to the varios models are shon in Table 2. Both the 1BCM and 2BCM ith first order absorption ere nable to accont for the rapid absorption phase and reslted in a large over-estimation of initial concentrations folloed by an inability to attain the high peak concentrations (Figs. 4 and 5). A rapid increase in drg concentration after oral administration folloed by a fairly rapid decline is reported to be characteristic of ero order absorption (McNamara et al., 1978). Therefore a model incorporating ero order absorption as sed in an attempt to improve characterisation of the absorption and immediate postabsorption phases. This reslted in a slight improvement of fit as indicated by the correlation coefficients hich increased from a mean of to 0.981, AIC vales (Table 3) and visal observation (Fig. 4). --' >: " 0 - «a:: >: ::J a:: (f) 25 a Sbject 2 - One absorpt;on rate constant --- To absorpt;on rate constants Zero-order absorpuon T I ME (HOURS) Fig. 4. Observed data for Sbject 2 and compter-predicted fits to the IBCM ith one absorption rate constant, to absorption rate constants and ero order absorption. The arro indicates the time chosen for the ka to change in the IBCM ith to k;s. :. " >: «a:: 0 >: ::J a:: (f) a t TI ME (HOURS). Sbject 2 - Oneabsorpt;onrate constant - -- To absorpt;onrate constants ~ "!"" '''---~ Fig. 5. Observed data for Sbject 2 and compter-predicted fits to the 2BCM ith one and to absorption rate constants. The arro indicates the time chosen for the k a to change in the 2BCM ith to k;s. TABLE 2 Mean pharmacokinetic parameters of the 7 models Model kat ka, ka3 ko V KIO tlag K12 K21 T (h-1) (h-1) (h-1) (I g/h) (liter) (h-1) (h) (h -1) (h-1) (h) Al A A A Bl B B

6 146 TABLE 3 AlC vales for individal sbjects Model Sbjects Al A A A B B B Hoever, on close inspection of individal data, the reslting crves ere not ell characterised. The inclsion of a lag-time parameter in the ero order absorption model had no advantageos effect on the fit. Data ere then fitted to the models incorporating discontinos absorption. Using models A2 and B2 (Fig. 1) ith to absorption rate constants, a significant improvement in goodness of fit as noted (Fig. 5) according to all criteria. Characterisation of absorption and distribtion as greatly improved and generally, sbstantial differences in AIC vales ere noted (Table 3). The F ratio test (Table 4) indicated statistically significant differences (P < 0.01) in all sbjects beteen the models incorporating a single absorption rate constant and those characterised by discontinos absorption. A third absorption rate constant as then introdced and all individal data sets fitted to this model. Hoever, althogh AIC vales ere loer, TABLE4 Reslts of statistical comparison of models sing the F-ratio test Models Sbjects compared ~ A1/A2 A2/A3 B1/B2 B2/B * * * * * * * * * * * * * * Statistically significant difference (P < 0.01). a statistically significant improvement ith this model, as indicated by the F ratio test, as observed in only one sbject (Sbject 4). Discssion A possible problem sing eqal eighting in non-linear least sqares regression is that too little emphasis ill be placed on very lo concentrations if the data extend over a ide range of concentrations. Data in this stdy ere initially fitted sing eqal eighting and compared to reslts obtained sing eighting done according to the reciprocals of the observations. No improvement of fit either to the high or lo concentrations as observed, and parameter estimates sing eqal eighting ere in closer agreement to vales calclated sing non-compartmental methods. Discontinos absorption processes in relation to linear pharmacokinetic models have been reported by Siiverkriip (1979) and Zimmerman (1983). Zimmerman sed a system of differential eqations in hich seqential sets of eqations ere sed to describe the absorption profile from time ero to infinity. The seflness of these eqations as demonstrated by obtaining excellent fits for some nsal absorption profiles of griseoflvin (Bates and Carrigan, 1975) and slfisoxaole (Kaplan et al., 1972). With both of these drgs, previos, less sccessfl attempts had been made to characterise the absorption sing ero order inpt. Excellent characterisation of the absorption phase reslted from the se of a biphasic, discontinos absorption model. Previos orkers sing rine data after the administration of a soltion of PPA sggested that the absorption of the drg cold be described as biphasic, consisting of a slo phase folloed by a mch faster phase (Wilkinson and Beckett, 1968a). In the present stdy the initial absorption phase, hich as a slo one, as folloed by an extremely rapid rate of absorption (Table 2). On the basis of their higher AIC vales, the 1BCM and 2BCM ith continos first order absorption and the 1BCM ith ero order absorption ere disregarded. Althogh models A3 and

7 147 B3, both ith 3 absorption rate constants, displayed the loest AIC vales, application of the F ratio test indicated no statistically significant difference beteen these models (A3 and B3) and those containing to absorption rate constants (A2 and B2). Both the IBCM and 2BCM ith to absorption rate constants ere fond to be eqally appropriate, since no significant statistical difference as apparent be.teen either of these models at the 99% significance level, as exemplified by the reslts of the F ratio test. The semilog plots of serm concentration vs time after completion of absorption appeared to consist of to components, hich old sggest the existence of a distribtion phase in a 2BCM (Fig. 6). From observation of the individal sets of ra data and comparing them ith the predicted fits, the disposition and elimination phases of the serm concentration-time crve ere, in most cases, more accrately described sing the 2BCM hich acconts for loss of drg from the central compartment by to processes, elimination from the central compartment and transfer from the central into the peripheral compartment. Hoever, intravenos data are necessary for the final elcidation of the disposition characteristics of a drg, and as only extravasclar data ere available in this stdy, the disposition of PPA into a IBCM or 2BCM can only be postlated. In smmary, it has been shon that PPA exhibits rather nsal absorption characteristics 200 -' " co 100 co co 20 ::0 W TIME 6 8 (HOURS) Fig. 6. Typical semilogarithmic plot of serm concentration vs time after oral administration of a 50 mg aqeos soltion of the drg (Sbject 2). and the drg's pharmacokinetic profile is ell described sing a discontinos absorption model. Acknoledgements One of s (I.K.) acknoledges financial spport from the Medical Research Concil of S.A. References Axelrod, J., Stdies on sympathomimetic amines. I. The biotransformation and physiological disposition of I-ephedrine and I-norephedrine. J. Pharmacal. Exp. Ther., 109 (1953) Bates, T.R. and Carrigan, P.J., Apparent absorption kinetics of micronied griseoflvin after its oral administration on single- and mltiple-dose regimens to rats as a corn oil-inater emlsion and aqeos sspension. J. Pharm. Sci., 64 (1975) Beckett, A.H. and Wilkinson, G.R., Urinary excretion of (-)- methylephedrine, (- )-ephedrine and (- )-norephedrine in man. J. Pharm. Pharmacal., 17 (1965) 107S-108S. Bernstein, E. and Diskant, B.M., Phenylpropanolamine: a potentially haardos drg. Ann. Emerg. Med.., 11 (1982) Boxenbam, H.G., Riegelman, S. and Elashoff, R.M., Statistical estimations in pharmacokinetics. J. Pharmacokin. Biopharm., 2 (1974) Dose, R., Haigh, J.M. and Kanfer, I., Determination of phenylpropanolamine in serm and rine by high-performance liqid chromatography. J. Pharm. Sci., 72 (1983) D. Dose, R., Biopharmacetics of phenylpropanolamine, Ph.D Thesis, Rhodes University, Grahamston, Soth Africa, Greenood, J., The case against phenylpropanolamine. Pharm. J., 230 (1983) Heimlich, K.R., MacDonnell, D.R., Flanagan, T.L. and O'Brien, P.D., Evalation of a sstained release form of phenylpropanolamine hydrochloride by rinary excretion stdies. J. Pharm. Sci., 50 (1961) Horoit, J.D., Hoes, L.G., Christophidis, N., Lang, W.J., Fennessy, M.R., Rand, M.J. and Lois, W.J., Hypertensive responses indced by phenylpropanolamine in anorectic and decongestant preparations. Lancet, i (1980) Kaplan, S.A., Weinfeld, R.E., Abro, C.W. and Leis, M., Pharmacokinetic profile of slfisoxaole folloing intravenos, intramsclar, and oral administration to moj.n. J. Pharm. Sci., 61 (1972) Kanfer, J., Haigh, J.M. and Dose, R., Phenylpropanolamine hydrochloride. In K. Florey (Ed.), Analytical Profiles of Drg Sbstances, Vol. 12, Academic, Ne York, 1983, p. 360.

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