The development of antiretroviral agents (ARVs) with novel mechanisms of action is critical to provide treatment options for HIV patients with drug-resistant virus. Inhibitors of the HIV integrase enzyme, which integrates the viral DNA strand into the host cellular DNA, represent a novel class of ARVs targeting an essential process for viral replication.
Elvitegravir is a potent, once-daily, strand transfer inhibitor presently in phase 3 clinical trials in multiclass experienced and/or resistant HIV-infected patients. Elvitegravir is metabolized by cytochrome P450 3A4 (CYP3A4) and also by glucuronidation via uridine glucuronosyl transferase 1A1/3.1 Once-daily coadministration with ritonavir 100 mg (elvitegravir/r), a mechanism-based inhibitor of CYP3A4, causes a 20-fold increase in elvitegravir plasma exposure over the dosing interval [area under the concentration-time curve over the dosing interval (AUCtau)]. In addition, mean elvitegravir trough concentration (Ctau or Ctrough), which was the best determinant of antiviral activity in a 10-day monotherapy study and a 48-week phase 2 study, is ∼10-fold above its in vitro protein binding adjusted 95% inhibitory concentration (IC95) elvitegravir/r 150/100 mg administration.2-4 In a phase 2 study in treatment-experienced HIV patients, elvitegravir/r demonstrated potent and durable reduction in HIV-1 viral load when coadministered with other active ARVs and was well tolerated.5,6
Chemokine receptor 5 (CCR5) antagonists inhibit the fusion of HIV with the host cell by blocking the interaction between the viral gp-120 glycoprotein and the CCR5 chemokine receptor.7,8 Maraviroc is the first CCR5 coreceptor antagonist approved in United States, European Union, and Canada for the treatment of HIV-infected patients. In randomized, double-blind, placebo-controlled phase 3 studies (MOTIVATE 1 and MOTIVATE 2) in adult treatment-experienced patients with R5 HIV-1 only, significantly more patients had undetectable viral load (<50 copies/mL) and increased CD4 counts compared to baseline to once and twice-daily maraviroc plus a background regimen compared with placebo plus a background regimen.9 Maraviroc is metabolized primarily by CYP3A4, with no significant contributions from other CYP isozymes (1A2, 2B6, 2C8, 2C9, 2C19, 2D6). At clinically relevant concentrations, maraviroc is unlikely to inhibit CYP enzymes.10 Additionally, maraviroc is a substrate for the membrane transporter, P-glycoprotein (Pgp).11
The contribution of CYP3A4 and Pgp to maraviroc disposition suggests the potential for interaction with other antiretrovirals, many of which are substrates, inhibitors, and/or inducers of these enzymes. Indeed, due to the substantially higher plasma exposures of maraviroc upon coadministration with potent CYP3A4 inhibitors, including several ritonavir-boosted protease inhibitors, a 50% dose reduction (from 300 mg twice daily to 150 mg twice daily) is recommended in such combinations. Conversely, a 100% dose increase (from 300 mg twice daily to 600 mg twice daily) is recommended when maraviroc is coadministered with potent CYP3A4 inducers in the absence of a potent CYP3A4 inhibitor.12
Elvitegravir and maraviroc are efficacious and well tolerated agents belonging to novel antiretroviral classes with antiviral activity against multiclass resistant HIV-1. Regimens containing combinations of these newer agents offer significant potential to address unmet medical needs, including treatment of patients with efficacy and/or safety/tolerability issues with other ARV classes. However, the known interactions of maraviroc with CYP3A4 inhibitors necessitate evaluation of its interaction with boosted elvitegravir to allow appropriate dosing recommendations. Thus, the pharmacokinetics (PK) and safety of elvitegravir, ritonavir, and maraviroc were determined in this study after multiple dose administration of elvitegravir/r (150/100 mg once daily) and maraviroc (150 mg twice daily) alone and in combination.
Subjects and Study Design
This was a phase 1, single center, open label, crossover study in healthy male and female (nonpregnant, nonlactating) subjects (aged 18-45 years, inclusive) performed at Charles River Laboratories (formerly Northwest Kinetics, Inc, Tacoma, WA). Eligible subjects were administered the randomized study treatments as shown in Figure 1. The study protocol and informed consent document were reviewed and approved by Aspire Independent Review Board Inc (La Mesa, CA), and subjects provided written informed consent before study participation. Major inclusion criteria were healthy subjects based on medical history/physical exams/laboratory evaluations, normal 12-lead electrocardiogram, normal renal function, creatinine clearance ≥80 mL/min, no evidence of HIV, hepatitis B virus, or hepatitis C virus infection, and use of at least 2 forms of contraception including an effective barrier method. Exclusion criteria included plasma and blood donation within 7 and 56 days of study entry, respectively; history of postural hypotension; history of cardiac disease; history of chronic liver disease or impairment; use of prescription drugs within 30 days of study drug dosing (except vitamins, acetaminophen, ibuprofen, and/or hormonal contraceptive).
In group 1, subjects were randomized to receive elvitegravir/r 150/100 mg once daily followed by elvitegravir/r 150/100 mg once daily plus maraviroc 150 mg twice daily or vice versa (n = 10 per sequence). In group 2, subjects were randomized to receive maraviroc followed by maraviroc plus elvitegravir/r or vice versa (n = 8 per sequence). Each study treatment was administered for 10 days, which was considered to be sufficient for all agents to reach steady state. All morning doses of study drugs were administered under the supervision of clinic staff. Because elvitegravir exposures are higher when taken with food, subjects received study drugs immediately after a standardized morning meal (roughly 400 kcal) with 240 mL (8 fluid ounces) of water. Morning doses of maraviroc were administered 1 hour before breakfast. Evening doses of maraviroc were administered 12 hours after the morning dose and 1 hour before or at least 2 hours after the evening meal. On the days of PK sampling, subjects fasted until after collection of the 4-hour postdose blood draw. Subjects were allowed to consume water as desired except 1 hour before and 2 hours after study drug dosing.
On days 10 and 20, after elvitegravir/r dosing and/or the morning dose of maraviroc, the PK of elvitegravir and ritonavir were evaluated in group 1 and maraviroc in group 2, with the following sampling scheme: group 1, elvitegravir/r and elvitegravir/r plus maraviroc: predose (0), 1, 2, 3, 3.5, 4, 4.5, 5, 6, 8, 10, 12, 18, and 24 hours after dosing of elvitegravir/r; group 2, maraviroc and maraviroc plus elvitegravir/r: predose (0), and 0.5, 1, 1.5, 2, 3, 4, 5, 6, 8, 10, and 12 hours after dosing of maraviroc. Elvitegravir was supplied as 150 mg tablets, ritonavir as 100 mg soft gelatin capsules (Norvir; Abbott Laboratories, Chicago, IL), and maraviroc as 150 mg tablets, all for oral administration. Blood samples were collected in Vacutainer tubes containing anticoagulant (spray-dried K2 EDTA [dipotassium ethylenediaminetetraacetic acid]) and inverted several times to mix the blood and the anticoagulant. Samples were kept on wet ice or refrigerated for ≤30 minutes and centrifuged for 10 minutes at 1200 RCF (relative centrifugal force; g force) in a refrigerated centrifuge set at ∼4°C to harvest plasma.
Elvitegravir and ritonavir plasma concentrations were determined using validated high-performance liquid chromatography/tandem mass spectrometry (HPLC/MS/MS) at Gilead Sciences, Inc (Durham, NC) with previously reported bioanalytical methods13; maraviroc concentrations were determined at Tandem Laboratories (West Trenton, NJ). The assay calibration curves were linear between 20 and 10,000 ng/mL for elvitegravir and 5 and 5000 ng/mL for ritonavir. Interassay precision (expressed as percent coefficient of variation) for elvitegravir and ritonavir were in the ranges of 2.1%-6.3% and 8.0%-11.6%, respectively. Interassay accuracy, expressed as percent bias (percent difference from nominal concentrations), for elvitegravir and ritonavir were in the ranges of −13.0% to −2.4% and −2.0 to 9.4%, respectively.
Plasma analyses of maraviroc were prepared using solid phase extraction and assayed using HPLC/MS/MS using validated methods. In brief, human plasma (0.050 mL) was mixed with [D5]-maraviroc internal standard in acetonitrile and vortex-mixed. After centrifugation, an aliquot (0.010-0.020 mL) was injected into the HPLC system. Chromatographic separation was achieved using a Fluophase perfluorophenyl phase (PFP) column (4.6 × 50 mm, 5 μm; Thermo Electron Corporation, Pittsburgh, PA) and a mobile phase consisting of 80:20 volume:volume acetonitrile:25 mM ammonium acetate in aqueous 0.2% formic acid, at a flow rate of 1 mL/min. The analytes were detected using an Applied-Biosystem Sciex API 4000 LC/MS/MS system (MDI SCIEX, Concord, Ontario, Canada) operating in positive TurboIonSpray mode. The transition ions m/z 269→155 and 283→169 were monitored for maraviroc and [D5]-maraviroc, respectively. The assay calibration curves were linear between 0.50 and 500 ng/mL and corresponding interassay percent coefficient of variation and percent bias were in the ranges of 3.6%-4.9% and −5.8% to 3.4%.
Pharmacokinetic parameters of elvitegravir, ritonavir, and maraviroc were estimated by application of a nonlinear curve-fitting software package (WinNonlin software, Professional Edition, Version 5.2; Pharsight Corporation, Mountain View, CA) using noncompartmental methods and included maximum observed plasma concentration (Cmax), time to reach maximum concentration (Tmax), AUCtau, calculated using the linear up-log down trapezoidal method; 0-24 hours for elvitegravir and ritonavir; 0 to 12 hours for maraviroc), elimination half-life (T1/2), and concentration at the end of dosing interval (Ctau).
A parametric (normal theory) analysis of variance using a mixed effects model appropriate for a crossover design was fit to the natural logarithmic transformation of AUCtau, Cmax, and Ctau of elvitegravir, maraviroc, and ritonavir. Sample size of the study was based on variability estimates of Cmax, AUCtau, and Ctau for elvitegravir (group 1) and Cmax and AUCtau for maraviroc (group 2), and accounted for potential drop-outs. The final sample size provided at least 90% power to conclude lack of PK alteration based on the expected ratio of geometric least squares means (GMR) of treatments (coadministered:alone) of 100% and associated 90% confidence interval (CI) range of 70%-143% for elvitegravir.
Given the known effect of CYP3A-inhibitors, including several boosted protease inhibitors, on maraviroc PK, maraviroc exposures were expected to increase upon coadministration with boosted-elvitegravir in the present study. The sample size of 12 subjects was sufficient such that if maraviroc exposure (Cmax and AUCtau) increased by 100%, the 90% CI boundaries were predicted to be 162%-247% for Cmax and 136%-295% for AUCtau with 80% probability. The 90% CI boundaries for lack of PK alteration of elvitegravir (70%-143%) were chosen based on the efficacy of elvitegravir/r in a phase 2 study that evaluated multiple elvitegravir doses (125 mg, 50 mg, and 20 mg) and the cumulative safety data at the 125 mg dose and higher doses evaluated in phase 1 studies. The 150/100 mg dose of elvitegravir/r is being evaluated in phase 3 studies. A 150 mg twice-daily dose of maraviroc was selected based on the anticipated effect of elvitegravir/r coadministration on maraviroc PK, consistent with other potent CYP3A4 inhibitors, and to estimate the magnitude of increase in maraviroc exposures.
The PK of ritonavir, used at a subtherapeutic dose as a PK booster, was evaluated using exploratory 90% CI boundaries of 70%-143%. Subjects with an evaluable PK profile for the ratio of coadministered to alone treatment pair were included in the PK analysis.
Thirty-six subjects were enrolled in the study (group 1: 20, group 2: 16) with mean age of 25 years (range: 18-44); 61% were male and 39% were female. Median body mass index for subjects at screening was 24.2 kg/m2 (range: 19.3-29.6 kg/m2). Twenty-four subjects (67%) were white, 10 (28%) were black, and 1 subject each was American Indian and Asian. Three subjects in group 1 and 5 subjects in group 2 discontinued prematurely from the study. Three discontinuations were due to protocol deviations arising from incorrect dosing or positive result for urine drug screen. Four subjects withdrew consent, and 1 subject withdrew due to an adverse event (AE).
In group 1, treatment-emergent AEs were observed in 8 (42%) and 4 (22%) subjects, respectively, after elvitegravir/r plus maraviroc and elvitegravir/r alone administration; nervous system disorders were observed in 1 (5%) and 2 (11%) subjects, respectively. In Group 2, 4 (27%) and 5 (36%) subjects, respectively, had treatment-emergent AEs during elvitegravir/r plus maraviroc and maraviroc alone dosing; nervous system disorders were observed in 0 and 2 (14%) subjects. The most common treatment-emergent AE was headache. While receiving elvitegravir/r plus maraviroc, 1 subject discontinued study drugs prematurely due to orthostatic (postural) hypotension (of moderate severity), an AE that has been observed in previous maraviroc studies.12 Most graded laboratory abnormalities were mild to moderate in severity. No deaths, serious AEs, or grade 3, or grade 4 AEs occurred in this study.
The plasma concentration-time profiles of elvitegravir after administration of multiple elvitegravir/r doses alone and in combination with maraviroc are presented in Figure 2A. Corresponding elvitegravir PK parameters are presented in Table 1. Peak elvitegravir concentrations were observed 4.5 hours after dosing and elvitegravir. T1/2 was similar across treatments (11.4 and 11.1 hours). The %GMR (90% CI) for elvitegravir plasma Cmax, AUCtau, and Ctau were contained within the lack of PK protocol-defined alteration bounds of 70%-143%, indicating that elvitegravir PK was unaffected by maraviroc coadministration.
The plasma concentration-time profiles of maraviroc after multiple dose administration alone and with elvitegravir/r are presented in Figure 2B, and maraviroc PK parameters are presented in Table 1. Maraviroc reached peak plasma concentrations 2.0 hours postdose in both treatments. The percent GMR for maraviroc plasma Cmax, AUCtau, and Ctau were 2-fold to 4-fold greater, confirming that maraviroc PK was substantially affected by elvitegravir/r coadministration.
Ritonavir plasma concentration-time profiles after multiple dose administration of elvitegravir/r alone and in combination with maraviroc are presented in Figure 2C, and PK parameters are presented in Table 1. Plasma profiles, Tmax (4.6 hours in both groups) and T1/2 (5.8 and 5.6 hours) of ritonavir were similar between the 2 treatments. The GMR (%) and 90% CI for ritonavir Cmax and AUCtau were within the protocol-defined exploratory lack of alteration boundaries of 70%-143%.
This randomized, 2-group, 2-way crossover study demonstrated that, as expected, coadministration of elvitegravir/r and maraviroc does not affect the PK of elvitegravir or ritonavir, but does result in a substantial increase in maraviroc plasma exposures. After administration of study treatments, AEs were grade 1 or 2 and no grade 3 or 4 AEs, or serious AEs were observed, indicating that these treatments were generally well tolerated.
Elvitegravir, ritonavir, and maraviroc are all metabolized by CYP3A4. The lack of inhibitory effect of maraviroc toward CYP enzymes demonstrated in vitro10 has been confirmed clinically based on the minimal impact of maraviroc coadministration on the exposures of midazolam, a CYP3A4 probe substrate, relative to the administration of midazolam alone.14 Additionally, maraviroc does not undergo clinically relevant interactions with agents metabolized by glucuronosyl transferase enzymes.14 Although maraviroc is a Pgp substrate, elvitegravir displays minimal to no interaction with Pgp, and coadministration with known Pgp inducers such as tipranavir does not affect elvitegravir PK.15 Taken together, these data indicate that the observed lack of alteration in elvitegravir exposures upon maraviroc plus elvitegravir/r coadministration is not unexpected. Likewise, maraviroc was not expected to affect ritonavir exposures either.
Elvitegravir/r has previously been shown to lack clinically relevant drug interactions with ARVs such as nucleos(t)ide reverse transcriptase inhibitors,16,17 several ritonavir-boosted protease inhibitors,18,19 and the novel nonnucleos(t)ide reverse transcriptase inhibitor etravirine,13 and concomitant medications including the proton-pump inhibitor omeprazole.20 In contrast, potent CYP3A4 inhibitors, including ritonavir-boosted protease inhibitors (except tipranavir/r), have been shown to substantially increase maraviroc exposures and therefore, a reduction in maraviroc dose from 300 mg to 150 mg twice daily is recommended when it is coadministered with these agents.21-23 Upon coadministration of elvitegravir/r 125/100 mg, the clearance of intravenously administered midazolam decreased ∼83% compared with administration alone, indicating that elvitegravir/r is a net CYP3A4 inhibitor.24 Thus, maraviroc exposures were expected to be higher when administered with elvitegravir/r.
The magnitude of increases in maraviroc AUC (2.86-fold) and Cmax (2.15-fold) are within the range of those observed when maraviroc is coadministered with other potent CYP3A4 inhibitors. In this regard, data with ritonavir-boosted protease inhibitors, in particular, are pertinent given the prevalence of their combined use with maraviroc in advanced HIV-infected patients. The highest increase in maraviroc AUCtau (9.8-fold) and Cmax (4.8-fold) were observed with saquinavir/ritonavir 1000/100 mg twice daily. Other ritonavir-boosted protease inhibitors commonly used in treatment-experienced HIV-infected patients also substantially increase maraviroc Cmax and AUCtau, including darunavir/r (4-fold and 3-fold, respectively), lopinavir/r (4-fold and 2-fold, respectively), and atazanavir/r (4.9-fold and 2.7-fold, respectively). Relatively lower increases in maraviroc Cmax and AUCtau have been observed when these ritonavir-boosted protease inhibitors and maraviroc are coadministered together with a known CYP3A4 inducer, such as efavirenz; for example, 2.5-fold and 1.25-fold, respectively, with maraviroc plus efavirenz plus lopinavir/r.25 With all of these agents and combinations, the recommended maraviroc dose is 150 mg twice daily. Clinical use of maraviroc and elvitegravir in treatment-experienced patients is expected to be in the context of boosted protease inhibitors.
In conclusion, coadministration of elvitegravir/r and maraviroc does not warrant dose changes for elvitegravir/r, but a reduction in maraviroc dose to 150 mg twice daily is recommended, consistent with its use with potent CYP3A inhibitors, including boosted-protease inhibitors (except tipranavir/r).
TThe authors gratefully acknowledge Manoli Vourvahis (Pfizer, CT) for thoughtful review of the article and contribution to the Methods section.
1. Ramanathan S, Wright M, West S, et al. Pharmacokinetics, metabolism, and excretion of ritonavir-boosted GS-9137 (Elvitegravir). Presented at: Eighth International Workshop on Clinical Pharmacology of HIV Therapy; April 16-18, 2007; Budapest, Hungary. Abstract 30.
2. Mathias AA, West S, Hui J, et al. Effect of increasing ritonavir doses on hepatic CYP3A activity and GS-9137 (elvitegravir) oral exposure. Presented at: Eighth International Workshop on Clinical Pharmacology of HIV Therapy; April 16-18, 2007; Budapest, Hungary. Abstract 53.
3. Ramanathan S, West S, Hui J, et al. Clinical pharmacokinetics of once-daily elvitegravir boosted by ritonavir versus atazanavir. Presented at: Ninth International Workshop on Clinical Pharmacology of HIV Therapy; April 7-9, 2008; New Orleans, LA. Abstract O18.
4. DeJesus E, Berger D, Markowitz M, et al. Antiviral activity, pharmacokinetics, and dose response of the HIV-1 integrase inhibitor GS-9137 (JTK-303) in treatment-naïve and treatment-experienced patients. J Acquir Immune Defic Syndr
5. Zolopa A, Lampiris H, Blick G, et al. The HIV integrase inhibitor elvitegravir (EVG/r) has potent and durable antiretroviral activity in treatment-experienced patients with active optimized background therapy (OBT) [H-714]. Presented at: Forty Seventh Interscience Conference on Antimicrobial Agents and Chemotherapy; September 17-20, 2007; Chicago, IL.
6. Zolopa A, Mullen M, Berger D, et al. The HIV integrase inhibitor GS-9137 demonstrates potent antiretroviral activity in treatment-experienced patients [143LB]. Presented at: Fourteenth Conference on Retrovirus and Opportunistic Infections; February 25-28, 2007; Los Angeles, CA.
7. Cocchi F, DeVico AL, Garzino-Demo A, et al. Identification of RANTES, MIP-1α, and MIP-1β as the major HIV-suppressive factors produced by CD8+ T cells. Science
8. Arenzana-Seisdedos, F, Virelizier, JL, Rousset D, et al. HIV blocked by chemokine antagonist. Nature
9. Gulick RM, Lalezari J, Goodrich J, et al. Maraviroc for previously treated patients with R5 HIV-1 infection. N Engl J Med
10. Hyland R, Dickins M, Collins C, et al. Maraviroc: in vitro assessment of drug-drug interaction potential. Br J Clin Pharmcol
11. Walker DK, Abel S, Comby P, et al. Species differences in the disposition of the CCR5 antagonist, UK-427,857, a new potential treatment for HIV. Drug Metab Dispos
12. Abel S, Back DJ, Vourvahis M. Maraviroc: pharmacokinetics and drug interactions. Antivir Ther
13. Ramanathan S, Kakuda TN, Mack, R, et al. Pharmacokinetics of elvitegravir and etravirine following coadministration of ritonavir-boosted elvitegravir and etravirine. Antivir Ther
14. Abel S, Russell D, Whitlock LA, et al. Effect of maraviroc on the pharmacokinetics of midazolam, lamivudine/zidovudine, and ethinyloestradiol/levonorgestrel in healthy volunteers. Br J Clin Pharmacol
. 2008;65(Suppl 1):19-26.
15. Mathias AA, Hinkle J, Shen G, et al. Effect of ritonavir-boosted tipranavir or darunavir on the steady-state pharmacokinetics of elvitegravir. J Acquir Immune Defic Syndr
16. Ramanathan S, Shen G, Cheng A, et al. Pharmacokinetics of emtricitabine, tenofovir, and GS-9137 following co-administration of emtricitabine/tenofovir disoproxil fumarate and ritonavir-boosted GS-9137. J Acquir Immune Defic Syndr
17. Ramanathan S, Shen G, Hinkle J, et al. Pharmacokinetics of co-administered ritonavir-boosted elvitegravir and zidovudine, didanosine, stavudine, and abacavir. J Acquir Immune Defic Syndr
18. Mathias AA, Hinkle J, Shen G, et al. Effect of ritonavir-boosted tipranavir or darunavir on the steady-state pharmacokinetics of elvitegravir. J Acquir Immune Defic Syndr
19. Ramanathan S, Mathias AA, Shen G, et al. Lack of clinically relevant drug-drug interaction between ritonavir-boosted GS-9137 (elvitegravir) and fosamprenavir/r [WEBEP014]. Presented at: Fourth International AIDS Society Conference on Pathogenesis, Treatment, and Prevention; 2007; Sydney, Australia.
20. Ramanathan S, Shen G, Hinkle J, et al. Pharmacokinetic evaluation of drug interactions with ritonavir-boosted HIV integrase inhibitor GS-9137 (elvitegravir) and acid reducing agents. Presented at: Eighth International Workshop on Clinical Pharmacology of HIV Therapy; April 16-18, 2007; Budapest, Hungary. Abstract 69.
21. Abel S, Jenkins TM, Whitlock LA, et al. Effects of CYP3A4 inducers with and without CYP3A4 inhibitors on the pharmacokinetics of maraviroc in healthy volunteers. Br J Clin Pharmacol
. 2008;65(Suppl 1):38-46.
22. Muirhead G, Abel S, Russell D, et al. An investigation of the effects of atazanavir and ritonavir boosted atazanavir on the pharmacokinetics of the novel CCR5 inhibitor UK-427,857. Presented at: 7th International congress on drug therapy in HIV infection [P283]; November 14-18, 2004; Glasgow, United Kingdom.
23. Abel S, Ridgway C, Hamlin J, et al. An open, randomized, 2-way crossover study to investigate the effect of darunavir/ritonavir on the pharmacokinetics of maraviroc in healthy subjects. Presented at: Eighth International Workshop on Pharmacology of HIV Therapy; April 16-18, 2007; Budapest, Hungary. Abstract 55.
24. Mathias AA, West S, Hui J, et al. Dose-Response of Ritonavir on Hepatic CYP3A Activity and Elvitegravir Oral Exposure. Clin Pharmacol Ther
25. Muirhead G, Ridgway C, Leahy D. A study to investigate the combined coadministration of P450 CYP3A4 inhibitors and inducers on the pharmacokinetics of the novel CCR5 inhibitor UK-427,857 [P284]. Presented at: Seventh International congress on drug therapy in HIV infection; November 14-18, 2004; Glasgow, United Kingdom.
Keywords:© 2010 Lippincott Williams & Wilkins, Inc.
CCR5; elvitegravir; interaction; integrase; maraviroc; pharmacokinetics