Pharmacokinetics of the Multi‐kinase Inhibitor Pexidartinib: Mass Balance and Dose Proportionality

Abstract

Pexidartinib is an oral small‐molecule tyrosine kinase inhibitor that selectively targets colony‐stimulating factor 1 receptor. Two phase 1 single‐center trials were conducted in healthy subjects to determine the absorption, distribution, metabolism, and excretion of pexidartinib using radiolabeled drug and to assess the dose proportionality of pexidartinib following single oral doses. In the mass balance study, eight male subjects received a single oral dose of [¹⁴C]‐pexidartinib 400 mg with radioactivity assessed in plasma, urine, and feces samples taken at various timepoints postdose. In the dose‐proportionality study, 18 subjects received single doses of pexidartinib 200, 400, and 600 mg using randomization sequences. Peak pexidartinib and total radioactivity were observed at 1.75–2.0 hours after the oral dose and then declined in a multiphasic manner. The overall mean recovery of administered radioactivity was 92.2% over 240 hours with 64.8% in the feces and 27.4% in the urine. Major components detected in plasma were pexidartinib and glucuronide (M5, ZAAD‐1006a), with M5 and pexidartinib detected in urine and feces, respectively. A glucuronide of dealkylated form (M1) in the urine and multiple oxidized forms (M2, M3, and M4) in feces were detected. The dose‐proportionality study found dose‐proportional drug exposure between the 200‐ and 400‐mg doses and slightly less than proportional exposure between the 400‐ and 600‐mg doses. These results from these studies provide insight into pexidartinib disposition after oral administration and support the development of dosing guidance in subjects with renal or hepatic impairment or subjects taking cytochrome P450 3A and uridine disphosphate‐glucuronosyl transferase inhibitors and inducers.

Pexidartinib is an oral small‐molecule tyrosine kinase inhibitor that selectively targets colony‐stimulating factor1 receptor, which is a tyrosine kinase transmembrane receptor for macrophage colony‐stimulating factor. Pexidartinib also inhibits KIT proto‐oncogene receptor tyrosine kinase and FMS‐like tyrosine kinase 3 harboring an internal tandem duplication mutation. ¹ The drug is approved by the US Food and Drug Administration for the treatment of adults with symptomatic tenosynovial giant cell tumor (TGCT) associated with severe morbidity or functional limitations and not amenable to improvement with surgery. It is available only through a restricted Risk Evaluation and Mitigation Strategy program due to hepatotoxicity risk. ² , ³

Following oral administration, pexidartinib is rapidly absorbed, with maximum plasma concentrations occurring at approximately 2.5 hours and exposure increasing with increasing dose. The drug undergoes extensive metabolism via oxidation and glucuronidation, and previous pharmacokinetic analyses have found that pexidartinib is a moderate inducer of cytochrome P450 (CYP) 3A and a weak inhibitor of CYP2C9. ⁴ The objectives of these two studies were to further characterize the absorption, distribution, metabolism, and excretion of pexidartinib using radiolabeled drug and to assess the dose proportionality of pexidartinib following single oral doses.

Materials and Methods

Study Designs

The studies were conducted in compliance with the ethical principles of the Declarations of Helsinki and the International Conference on Harmonisation consolidated Guideline E6 for Good Clinical Practice and approved by the applicable institutional review boards. Each study was conducted at a single site in the United States with detailed information summarized in Table S1. All subjects provided written informed consent prior to study participation.

Mass Balance Study

The mass balance study was a phase 1, single‐center, open‐label, nonrandomized study in healthy male subjects. After a screening period of up to 20 days, eligible participants were admitted into the clinical site from day −1 to at least day 7. On day 1, each subject received a single oral dose of [¹⁴C]‐pexidartinib 400 mg (150 μCi) as a suspension with approximately 50 mL of water after an overnight fast of at least 8 hours. Subjects continued to fast for at least 4 hours after the administration of study drug. The participants remained at the clinical site for at least 168 hours postdose with discharge on day 8 if >90% of the radioactivity was recovered and <1% of the radioactive dose was recovered in urine and feces over two consecutive 24‐hour collection periods. If these criteria were not met, subjects remained at the study site until day 11.

Dose‐proportionality Study

This was a phase 1, open‐label, randomized three‐period crossover study. After a 20‐day screening period (day −21 through day −2), eligible subjects were domiciled at the study clinic on days −1 through day 30. During each 10‐day treatment period, subjects received a single oral dose of pexidartinib on day 1 followed by a 10‐day washout period. Subjects were randomized to one of three treatment sequences: ABC, BCA, or CAB, where treatments A, B, and C consisted of a single oral dose of pexidartinib 200, 400, and 600 mg, respectively, on day 1 of each period. Pexidartinib was administered with 240 mL of water in the morning after an overnight fast of at least 8 hours, followed by a fast for an additional 4 hours.

Subjects

Inclusion/Exclusion Criteria

The studies included healthy subjects aged 19–55 years with a body mass index (BMI) of 19–32 kg/m², inclusive (mass balance study), or 18–60 years with a BMI of 18–30 kg/m², inclusive (dose‐proportionality study). The mass balance study only included male subjects while the dose‐proportionality study included both males and females. In both studies, subjects were required to have a negative urine test for drugs of abuse, HIV antibody, hepatitis B surface antigen, and hepatitis C virus antibody. Male subjects were either surgically sterile or agreed to use double‐barrier methods of contraception. Female subjects in the dose‐proportionality study were required to have a negative serum pregnancy test, be nonlactating and be either surgically sterile or naturally postmenopausal for at least 12 consecutive months.

Subjects with a history of stomach or intestinal surgery or resection that may alter drug absorption and/or excretion of study medication were excluded from participation in both studies as were individuals with a history or current evidence of clinically significant cardiac, hepatic, renal, pulmonary, endocrine, neurologic, infectious, gastrointestinal, hematologic, or oncologic disease or any other medical disorders or conditions that may prevent successful completion of the study. In addition, the use of any medications known to induce or inhibit cytochrome P450 (CYP)3A4 or CYP2C9 within 28–30 days prior to the study and use of any prescription or over‐the‐counter medications within 14 days prior to the study (except acetaminophen and topical hydrocortisone cream for contact dermatitis) were prohibited.

Procedures

Mass Balance Study

Radioactivity was determined for plasma, urine, and feces over each collection period (Drug Metabolism and Pharmacokinetics Department, Covance Laboratories, Madison, Wisconsin) with urine and fecal radioactivity data evaluated for total excretion and percentage of dose (ie, mass balance). Regarding pharmacokinetics and radioactivity, blood samples were collected at 0 hours (predose), followed by 0.5, 1, 1.5, 2, 2.5, 3, 3.5, 4, 4.5, 5, 6, 8, 12, 16, 24, 36, 48, 60, 72, 96, 120, 132, 144, 156, and 168 hours postdose. Blood samples were collected at 192, 216, and 240 hours postdose only if radioactivity discharge criteria had not been met. Metabolite blood samples were pulled at predose and at 0.5, 1, 2, 3, 4, 6, 12, 24, 48, and 72 hours postdose. Urine samples were collected at predose (−2 to 0 hours) and subsequently at 0–4, 4–8, 8–12, 12–24, 24–36, 36–48, 48–72, 72–96, 96–120, 120–144, and 144–168 hours postdose. If the radioactivity discharge criteria were not met, urine samples were then collected at 168–192, 192–216, and 216–240 hours postdose. Fecal samples were collected at predose (−24 to 0 hours), followed by 0–24, 24–48, 48–72, 72–96, 96–120, 120–144, and 144–168 hours postdose. Similar to urine sample collection, fecal samples were pulled at 168–192, 192–216, and 216–240 hours postdose if the radioactivity discharge criteria had not been met.

Radioactivity in each pooled plasma sample was determined by liquid scintillation counting (LSC). Plasma samples were sonicated, vortex mixed (with acetonitrile), and centrifuged, and duplicate aliquots were analyzed by LSC to determine reconstitution recoveries. The reconstituted samples were analyzed by liquid chromatography‐mass spectrometry (LC‐MS) with eluent fractions collected at 10‐second intervals into 96‐well plates containing solid scintillant. Radioactivity in urine samples was determined by LSC. Pooled urine was analyzed by LC‐MS with eluent fractions collected at 10‐second intervals into 96‐well plates containing solid scintillant. The pooled feces sample was sonicated, vortex mixed (with acetonitrile), and centrifuged, and the supernatant was extracted. Radioactivity in plasma, urine, and fecal samples was determined using TopCount NXT analysis, and radiochemical profiles were generated based on radioactivity counts. The percentage of radioactivity associated with red blood cells was determined by comparing the concentration of radioactivity in whole blood and plasma. Plasma concentrations of pexidartinib and its primary N‐glucuronide metabolite (ZAAD‐1006a) were analyzed by the separate bioanalytical methods previously reported. ⁵ Pexidartinib was analyzed using a validated liquid chromatography‐tandem mass spectrometry (LC‐MS/MS) method and ZAAD‐1006a was analyzed using a qualified LC‐MS/MS method.

Dose‐proportionality Study

Serial plasma samples were collected at the following timepoints for the pharmacokinetic analyses of pexidartinib and ZAAD‐1006a: 0.5, 1, 1.5, 2, 2.5, 3, 3.5, 4, 4.5, 5, 6, 8, 10, 12, 24, 36, 48, 72, 96, 120, and 144 hours postdose. Plasma samples were analyzed for the quantification of pexidartinib using a validated LC‐MS/MS method as previously described. ⁵

Safety

Safety endpoints included treatment‐emergent adverse events (TEAEs), physical examination, vital signs, 12‐lead electrocardiogram, and laboratory assessments. Adverse events were assessed by the investigator for severity and relationship to study medication. The elemental composition of the five metabolites was confirmed using accurate mass spectrometry analysis.

Data Analysis

The pharmacokinetic analysis set for both studies included all subjects who received a dose of pexidartinib and had sufficient plasma concentration data to characterize pharmacokinetic parameters, and the safety analysis set included all subjects who received at least one dose of pexidartinib.

Mass Balance Study

The net amount of radioactivity in each peak was expressed as a percentage of total radioactivity in the chromatogram or sample. Metabolite profiles in plasma were reported as a percentage of total sample radioactivity and as concentration (ng equivalents/g).

For excreta, the percentage of the administered dose excreted as the component represented by the peak was calculated using the following equation.

$$\begin{array}{ccc}\hfill \%\mathrm{of}\mathrm{dose}& =& \left(\%\mathrm{of}\mathrm{radioactivity}\mathrm{in}\mathrm{peak}/100\right)\hfill \\ & & \times \%\mathrm{of}\mathrm{dose}\mathrm{in}\mathrm{sample}\hfill \end{array}$$

The percentage dose (for excreta) and concentration (for plasma) of the peak were corrected for extraction and reconstitution recoveries, as applicable.

Metabolite numbers (eg, M1, M2) were assigned in retention time order (ie, M1 through M5). In the radiolabel study, quantification of the metabolites present in the plasma, urine, and feces was based on the profiles of radioactivity. The limit of quantitation for radioactivity in all matrices was 1% of the total run or less than 10 cpm peak height. Radioactive peaks less than this threshold were reported as not detected. Later the relative abundance of each analyte, parent, and metabolites was calculated based on the radioactivity.

Where possible, pharmacokinetic parameters were determined from the plasma concentrations of pexidartinib and ZAAD‐1006a, and from the blood and plasma concentrations of total radioactivity. Noncompartmental methods were performed using Phoenix WinNonlin (Certara USA, Inc., Princeton, New Jersey) Version 6.4. Pharmacokinetic parameter data for pexidartinib and ZAAD‐1006a in plasma, and for total radioactivity in plasma and whole blood included maximum plasma concentration (C_max), area under the drug plasma concentration–time curve until the last measurable concentration (AUC_last), AUC from time 0 to infinity (AUC_inf), time to reach C_max (T_max), and half‐life (t_1/2).

Dose‐proportionality Study

Pharmacokinetic parameters for pexidartinib and M5 (ZAAD‐1006a) were computed using WinNonlin Professional (Version 6.4). Plasma concentration values that were below the limit of quantification (BLQ) at the beginning and end of a pharmacokinetic profile were set to zero, provided that the BLQ values were not flanked by measurable concentrations. BLQ values that were flanked at adjacent times by measurable concentrations were set to missing in the pharmacokinetic calculations. Pharmacokinetic endpoints included C_max, T_max, AUC_last, AUC_inf, and t_1/2 for pexidartinib and ZAAD‐1006a.

Dose proportionality was evaluated by (1) graphical plots, (2) analysis of variance (ANOVA) of dose‐normalized pharmacokinetic parameters, and (3) a power model. Graphical plots display the dose‐normalized C_max and AUC_last versus the pexidartinib dose for the pharmacokinetic analysis set. The ANOVA analysis used a mixed‐effects model with sequence, treatment, and period as fixed effects, and subjects as random effect on the dose‐normalized ln‐transformed AUC_last and C_max. Dose proportionality was assessed by comparing the AUC_last and C_max values of the 400‐mg reference dose to those seen with the 200‐mg and 600‐mg doses of pexidartinib. Point estimates (ie, geometric mean) of each treatment and geometric least‐square mean ratios and 90% confidence interval (CI) were calculated. The predefined boundary of bioequivalence was 80%–125%.

The power model involved the ln‐transformation of the C_max and AUC, and was described as:

ln(Y) = β0 + β ln dose + ε

where β is the slope and Y is the pharmacokinetic parameter.

Dose proportionality was concluded if the 2‐sided 90%CI of the slope β was within the predefined 80% to 120% acceptance interval calculated as [1 + ln(L)/ln(R), 1 + ln(H)/ln(R)], where L = 0.8, H = 1.25, and R is the ratio of the highest dose to lowest dose. Dose proportionality was ruled out if the calculated 90%CI was completely outside the predefined 80%–120% acceptance interval. Dose proportionality was considered “inconclusive” when the 90%CI overlapped the 80%–120% acceptance interval.

Adverse events were coded using MedDRA (version 19.0). Safety parameters for both studies were reported descriptively.

In Vitro Metabolic Enzyme Identification Study

To identify human CYP and uridine disphosphate‐glucuronosyl transferase (UGT) involved in metabolism of pexidartinib, 1 μM pexidartinib was incubated with recombinant human CYP ([rCYP] 1A2, 2B6, 2C8, 2C9, 2C19, 2D6, 3A4, and 3A5) in the presence of nicotinamide adenine dinucleotide phosphate at 37°C for 30 minutes or recombinant human UGT ([rUGT] 1A1, 1A3, 1A4, 1A6, 1A9, 1A10, 2B4, 2B7, 2B10, 2B15, and 2B17) in the presence of uridine disphospho‐gluronic acid at 37°C for 120 minutes. Samples were analyzed by LC‐MS/MS.

Results

Subjects

In the mass balance study, eight male subjects were enrolled and all completed the study. Subjects were predominantly white (6/8 [75%]), with a mean age of 36.3 years and a mean BMI of 26.9 (3.76) kg/m² (Table 1). In the dose‐proportionality study, 18 subjects were enrolled and all completed the study. Subjects were predominantly male (15/18 [83%]) and Black/African American (13/18 [72%]), with a mean age of 41.6 years, mean weight of 82.2 kg, and mean BMI of 26.4 kg/m².

Table 1.

Subject Characteristics

Parameter	Mass Balance Study (N = 8)	Dose‐Proportionality Study (N = 18)
Male, n (%)	8 (100)	15 (83.3)
Mean age, years (SD)	36.3 (8.63)	41.6 (9.54)
Race
White	6 (75)	4 (22.2)
Black/African American	2 (25)	13 (72.2)
Other	0	1 (5.6)
Mean BMI, kg/m2 (SD)	26.9 (3.76)	26.4 (2.54)

BMI, body mass index; SD, standard deviation.

Pharmacokinetics

Mass Balance Study

Following oral administration, the overall mean recovery of administered radioactivity was 92.2% over 240 hours, with 64.8% recovered in the feces and 27.4% recovered in the urine. The percentage of radioactivity of pexidartinib and metabolites in the urine and feces is summarized in Table S2. Recovery of >90% radioactivity indicates that the ¹⁴C‐absorption, distribution, metabolism, and excretion (¹⁴C‐ADME) study was of adequate quality to draw conclusions on pexidartinib disposition.

Following a single oral dose of [¹⁴C]‐pexidartinib, radioactivity was rapidly detected in both blood and plasma with median (range) T_max values of 1.75 (1.0, 3.0) hours for pexidartinib in the plasma and 2.0 (1.5, 3.0) hours for total radioactivity in both plasma and whole blood. Thereafter, pexidartinib and total radioactivity steadily declined in a generally multiphasic manner.

The mean C_max for total plasma radioactivity (5591 ng equivalents/g) was approximately 2‐fold that of plasma pexidartinib (2915 ng/mL). The mean AUC from time 0 to 24 hours postdose (AUC_0‐24) for total plasma radioactivity was 67 092 h • ng/mL which was approximately 2‐fold higher than that of plasma pexidartinib (27 697 h • ng/mL). Similarly, the mean AUC_last for total radioactivity in plasma was 167 679 h • ng/mL, which was 3‐fold that of pexidartinib in plasma (60,432 h • ng/mL). In addition, the mean t_1/2 of total radioactivity in the plasma was much longer (46.9 hours) than that of either pexidartinib (28.7 hours) or M5 (ZAAD‐1006a) (28.2 hours) (Table S3). The AUC_inf‐based M:P ratio for ZAAD‐1006a (corrected for the molecular weight of the parent vs metabolite) was 1.10.

Of the metabolites, M5 (ZAAD‐1006a) was the most abundant radioactive component in pooled urine, representing 10.3% of the total dose. Metabolites M5 and M1 were detected in the urine (10.3% and 5.4% of total dose, respectively) whereas there was no detectable pexidartinib in the urine (Table S2). In the feces samples, pexidartinib and three metabolites (M2, M3, M4) were detected, representing 44.0%, 2.4%, 0.93%, and 0.82% of the total administered radioactivity, respectively (Table S2). The five metabolites were identified to be a glucuronide of dealkylated form (M1), a dihydrodiol form (M2), oxidized forms (M3, M4), and a glucuronide (M5) (Figure 1).

Figure 1.

Proposed biotransformation pathways of pexidartinib in humans. f, feces; p, plasma; u, urine; 1, oxidation; 2, epoxidation; 3, epoxide hydrolysis; 4, oxidative dealkylation; 5, glucuronidation. Pathways are proposed based on general knowledge of metabolism and do not imply definitive pathways. Direct experimentation was not performed.

Dose‐proportionality Study

The mean plasma pexidartinib concentration–time curves are illustrated in Figure 2 and have consistent pharmacokinetic profiles across the evaluated dose levels. Peak plasma concentrations of pexidartinib were observed at 2.5 hours for all three doses, and the mean half‐life ranged between 24.8 and 26.7 hours over the dose range. Pharmacokinetic parameters are summarized in Table 2 and show that drug exposure (ie, C_max, AUC) increased with increased doses, although the increase was slightly less than dose proportional between 400 and 600 mg. In the graphical plots of C_max versus pexidartinib dose, the slope estimate was 0.83 (90%CI 0.74, 0.91) while the slope estimate for AUC_last was 0.85 (90%CI 0.77, 0.92).

Figure 2.

Plasma concentration–time profile of pexidartinib by dose: (A) linear scale and (B) semi‐logarithmic scale. Mean concentrations <10.0 ng/mL are not presented in the plot as the LLOQ is 10.0 ng/mL. LLOQ, lower limit of quantification; SD, standard deviation.

Table 2.

Summary of Plasma Pexidartinib Pharmacokinetic Parameters by Dose

Pharmacokinetic Parameter	Statistic	Pexidartinib 200 mg (n = 18)	Pexidartinib 400 mg (n = 18)	Pexidartinib 600 mg (n = 18)
Pexidartinib
Cmax (ng/mL)	Mean (SD)	2487 (842)	4657 (1503)	6027 (1800)
Tmax (h)	Median (range)	2.5 (1.5, 3.0)	2.5 (1.5, 8.0)	2.5 (1.5, 4.5)
AUClast (h • ng/mL)	Mean (SD)	43 307 (14 947)	85 276 (29 591)	107 438 (35 251)
AUCinf (h • n/mL)	Mean (SD)	44 542 (15 769)	87 891 (31 462)	110 211 (37 539)
t1/2 (h)	Mean (SD)	25.5 (5.2)	26.7 (6.5)	24.8 (6.0)
M5 (ZAAD‐1006a)
Cmax (ng/mL)	Mean (SD)	2053 (1012)	3769 (1741)	5536 (2986)
Tmax (h)	Median (range)	4 (2.5, 5.0)	4.5 (3.0, 8.0)	4.5 (2.5, 10.0)
AUClast (h • n/mL)	Mean (SD)	63 022 (34 945)	116 302 (52 352)	153 771 (76 545)
AUCinf (h • n/mL)	Mean (SD)	65 098 (37 419)	119 572 (54 743)	157 268 (78 953)
t1/2 (h)	Mean (SD)	25.0 (4.5)	26.4 (6.1)	24.7 (5.0)

AUC_inf, area under the plasma concentration‐time curve from time 0 to infinity; AUC_last, area under the plasma concentration–time curve from time 0 to the last measurable concentration; C_max, maximum plasma concentration; h, hours; SD, standard deviation; t_1/2, terminal elimination half‐life; T_max, time to C_max.

In the ANOVA analysis comparing 200‐ and 400‐mg doses of pexidartinib, the 90%CIs for the ratios of geometric LS means for dose‐normalized C_max, AUC_last, and AUC_inf were within the 80%–125% interval (Table 3), indicating dose proportionality between these doses. For the comparison of the 400‐ and 600‐mg doses, the 90%CI for the ratio of geometric LS means for dose‐normalized C_max, AUC_last, and AUC_inf overlapped with the 80%–125% interval, with ratios ranging from 84% to 86% for these three variables. The power model showed generally similar results. The slope estimate for AUC_last across all three dose levels was 0.85 (90%CI 0.77, 0.92) and it was 0.83 (90%CI, 0.74, 0.91) for C_max. Since the 90%CIs for AUC_last and C_max overlapped the 0.80–1.20 acceptance interval, dose proportionality was inconclusive.

Table 3.

ANOVA Model Analyses of Plasma Pexidartinib Pharmacokinetic Parameters

		Pexidartinib 200 mg vs 400 mg		Pexidartinib 400 mg vs 600 mg
Pharmacokinetic parameter	Statistic	Pexidartinib 200 mg (n = 18)	Pexidartinib 400 mg (n = 18)	Pexidartinib 400 mg (n = 18)	Pexidartinib 600 mg (n = 18)
Cmax (ng/mL • mg)	Geometric LS mean	11.7	11.0	11.0	9.5
	Geometric LS mean ratio (90%CI)	105.9 (96.8, 115.8)		86.3 (79.0, 94.4)
AUClast (ng • h/mL • mg)	Geometric LS mean	204.1	201.4	201.4	169.4
	Geometric LS mean ratio (90%CI)	101.3 (93.5, 109.8)		84.1 (77.6, 91.2)
AUCinf (ng • h/mL • mg)	Geometric LS mean	209.4	206.8	206.8	173.2
	Geometric LS mean ratio (90%CI)	101.3 (93.5, 109.7)		83.7 (77.3, 90.7)

AUC_inf, area under the plasma concentration‐time curve from time 0 to infinity; AUC_last, area under the plasma concentration‐time curve from time 0 to last measurable concentration; CI, confidence interval; C_max, maximum plasma concentration; h, hours; LS, least squares.

Safety

Safety results were generally consistent with the previously defined safety profile of pexidartinib. No subject experienced a TEAE leading to death, serious adverse event, or discontinuation in either of the studies. In the mass balance study, there were four TEAEs, including constipation (n = 2), diarrhea (n = 1), and erythema (n = 1), with all events considered mild (grade 1). Diarrhea was the only TEAE deemed by the investigator to be related to pexidartinib treatment.

In the dose‐proportionality study, four subjects experienced TEAEs including rhinitis (n = 2) and skin abrasion, headache, epistaxis, and pruritus (n = 1 each). All TEAEs were grade 1, and only one subject had adverse events (pruritus, followed by skin abrasion) that were considered related to study drug administration.

In Vitro Metabolic Enzyme Identification Study

Following incubation with rCYPs, pexidartinib was extensively metabolized by rCYP3A4 and rCYP3A5 (100% loss). When pexidartinib was incubated with a panel of rUGTs, the primary enzyme responsible for substrate loss was rUGT1A4 (25% loss). These data indicate that CYP3A4 and UGT1A4 are the main metabolizing enzyme contributing to metabolism (data on file).

Discussion

Pexidartinib is a multikinase inhibitor that has demonstrated antitumor activity in a variety of malignancies, including TGCT. ¹ , ³ , ⁶ , ⁷ The basic pharmacokinetic profile of pexidartinib has been described, ⁸ but the current manuscript provides a more detailed analysis of pexidartinib pharmacokinetics, including the main pathway for disposition, the major metabolite, and dose proportionality, that has not been previously published. The mass balance study provides a profile of the absorption, distribution, metabolism, and excretion of pexidartinib after oral administration and a detailed description of the metabolic fate of the drug. After a single dose of radioactive pexidartinib, the drug is rapidly absorbed (T_max = 1.75 hours in the plasma) and steadily eliminated (t_1/2 = 28.7 hours). Pexidartinib has a moderate oral bioavailability as 44% of the administered dose was excreted as unchanged drug in the feces.

Overall, over 92% of the radioactive dose was recovered over 240 hours, with most of the administered radioactivity recovered in the first 120 hours postdose. The results demonstrate that the feces are the major route of elimination of pexidartinib, with 64.8% of the radioactive dose recovered in the feces compared with 27.4% recovered in the urine. Furthermore, pexidartinib is highly metabolized, as indicated by the presence of only 34.9% of the total radioactivity as the parent compound in the plasma. The longer t_1/2 of plasma radioactivity as compared to pexidartinib plasma t_1/2 indicates the presence of metabolites with longer half‐lives in circulation. However, apart from ZAAD‐1006a, which had a similar half‐life to pexidartinib, no other major metabolites were identified in plasma. M4 was the only other metabolite present in the plasma albeit at low concentrations (<4%). This suggests that there could be multiple minor metabolites in plasma. The metabolites were considered minor as if the radioactivity associated with any peak was more than 1% of the total radioactivity in the run, the peak was characterized for the structure of the molecule. Their relative exposure was <1% of the total dose since 1% was the cutoff for metabolite characterization. Overall, theproposed pathways of biotransformation include multiple oxidations, epoxidation, epoxide hydrolysis, dealkylation, and glucuronidation (Figure 1). M5 (ZAAD‐1006a), an N‐glucuronide, was the primary metabolite identified in the plasma, with M4 being the only other metabolite present in the plasma, albeit at low concentrations (<4%).

The ¹⁴C‐ADME study was critical as it characterized the disposition of pexidartinib, including but not limited to identification of the elimination pathway, extent of metabolism and identification of the major metabolite. The findings from the ¹⁴C‐ADME study were key to designing the clinical pharmacology program (eg, the need for studies in special population like subjects with renal or hepatic impairment, characterization of the major metabolite, drug‐drug interaction [DDI] risk assessment). Results from the ¹⁴C‐ADME study suggested that pexidartinib is highly metabolized, hence a study was conducted to investigate the effect of hepatic impairment on pexidartinib pharmacokinetics. ⁵ As >20% of administered dose was eliminated renally, a renal impairment study was conducted to derive the dosing recommendation in subjects with renal impairment. ⁸

Additionally, once the metabolic pathway was postulated based on the metabolite profile in this study, DDI studies were conducted to evaluate the effect of CYP3A4 and UGT inhibitors and inducers on pexidartinib pharmacokinetics. ⁴ Lastly, this study identified ZAAD‐1006a as the major human plasma metabolite and its exposure in humans. As a follow‐up, toxicological studies in monkeys were initiated since human equivalent ZAAD‐1006a exposure could only be achieved in monkeys. More specifically, in the monkey toxicity study (data on file), no significant safety issues were observed at an exposure of the metabolite similar or higher than the predicted steady‐state exposure in patients with TGCT. It was therefore concluded that the metabolite was not associated with any toxicity at an exposure similar to or higher than the predicted steady‐state exposure in patients with TGCT. In addition, ZAAD‐1006a was evaluated for the in vitro DDI risk assessment and for target and nontarget pharmacological activity in vitro kinase assay. The results from all these clinical pharmacology studies were further utilized to derive dosing recommendations for pexidartinib.

In the dose‐proportionality study, the results indicated dose proportional pharmacokinetics of pexidartinib over the dose range of 200 mg to 400 mg. However, there was slightly less than a dose‐proportional increase in exposure over the 400–600‐mg range, with LS mean ratios ranging from 0.83 to 0.87 for C_max and AUC_inf. The dose‐proportional pharmacokinetics of pexidartinib between 200 and 400 mg has simplified the pexidartinib dose adjustment recommendation in renal impaired subjects and when coadministered with a strong and moderate CYP3A inhibitor and UGT inhibitor. Based on the extent of the impact of each of these factors on pexidartinib exposure, a proportionally reduced dose is recommended in subjects with renal impairment and when coadministered with strong and moderate CYP3A inhibitor and with UGT inhibitor. For patients with mild to severe renal impairment (ie, creatinine clearance 15–89 mL/min), the recommended dose of pexidartinib is 200 mg in the morning and 400 mg in the evening. For those who cannot avoid the use of concomitant moderate or strong CYP3A inhibitors or UGT inhibitors, the pexidartinib dose should be reduced to 200 mg twice daily for those with a planned total daily dose of 600 or 800 mg and reduced to 200 mg once daily for those with a planned total daily dose of 400 mg. ⁹

As a whole, these data demonstrate that pexidartinib is extensively metabolized, mainly via CYP3A4 and UGT, and is excreted predominantly via the feces as an unchanged molecule. These results are consistent with in vitro studies using recombinant human CYP/UGT demonstrating that pexidartinib is mainly metabolized by CYP3A4/3A5 and UGT1A4 (data on file). Therefore, drugs that modulate CYP3A4 and UGT activity may impact pexidartinib exposure and subsequently clinical activity. In addition, the dose‐proportionality results provide guidance on dosage adjustments for those with renal impairment or taking moderate/strong CYP3A inhibitors or UGT inhibitors. The ¹⁴C‐ADME and dose proportionality studies are critical studies describing the key pharmacokinetic characteristics of pexidartinib, enabling optimal dosing of the drug in special populations and when administered with concurrent medications.

Conflicts of Interest

Hamim Zahir reports employment with Daiichi Sankyo, Inc. during the time of the study. Jonathan Greenberg reports employment with and stock/stock options in Daiichi Sankyo, Inc. Ching Hsu reports employment and stock/stock options in Daiichi Sankyo, Inc. Kengo Watanabe reports employment with Daiichi Sankyo, Co., Ltd. Chie Makino reports employment with Daiichi Sankyo. Co., Ltd. Ling He reports employment with Daiichi Sankyo, Inc. Frank LaCreta reports employment with and stock/stock options in Daiichi Sankyo, Inc.

Supporting information

Supporting Information