Effects of Quinidine or Rifampin Co‐administration on the Single‐Dose Pharmacokinetics and Safety of Rilzabrutinib (PRN1008) in Healthy Participants

This open‐label, phase 1 study was conducted with healthy adult participants to evaluate the potential drug‐drug interaction between rilzabrutinib and quinidine (an inhibitor of P‐glycoprotein [P‐gp] and CYP2D6) or rifampin (an inducer of CYP3A and P‐gp). Plasma concentrations of rilzabrutinib were measured after a single oral dose of rilzabrutinib 400 mg administered on day 1 and again, following a wash‐out period, after co‐administration of rilzabrutinib and quinidine or rifampin. Specifically, quinidine was given at a dose of 300 mg every 8 hours for 5 days from day 7 to day 11 (N = 16) while rifampin was given as 600 mg once daily for 11 days from day 7 to day 17 (N = 16) with rilzabrutinib given in the morning of day 10 (during quinidine dosing) or day 16 (during rifampin dosing). Quinidine had no significant effect on rilzabrutinib pharmacokinetics. Rifampin decreased rilzabrutinib exposure (the geometric mean of Cmax and AUC0‐∞ decreased by 80.5% and 79.5%, respectively). Single oral doses of rilzabrutinib, with or without quinidine or rifampin, appeared to be well tolerated. These findings indicate that rilzabrutinib is a substrate for CYP3A but not a substrate for P‐gp.

Bruton's tyrosine kinase (BTK) is an essential element of signaling pathways downstream of the B-cell receptor (BCR), the Fc-gamma receptor, and the Fc-epsilon receptor. 1,2The inhibition of BTK activity in B cells produces phenotypic changes similar to BCR blockade.0][11] Due to its slow off-rate it has prolonged BTK occupancy and an ensuing long duration of action. 3,9,10The free, unbound form of rilzabrutinib has a short plasma half-life (approximately 3-4 hours) while its bound form remains attached and active for long periods of time. 10Currently, several clinical trials of rilzabrutinib are ongoing, including a phase 3 randomized controlled trial (NCT04562766) in persistent/chronic immune thrombocytopenia (ITP) and 4 randomized controlled phase 2 trials (NCT04520451 in IgG4-related disease, NCT05104892 in moderate-tosevere asthma, NCT05107115 in moderate-to-severe chronic spontaneous urticaria, and NCT05002777 in warm autoimmune hemolytic anemia; Table S1).
Rilzabrutinib (SAR444671) was well tolerated in various phase 1 trials conducted in healthy participants. 10,12A thorough QTc study in healthy participants demonstrated that ascending doses of rilzabrutinib ranging from 50 to 1200 mg and multiple doses ranging from 300 mg once daily, 300 mg twice daily, and 450 mg twice daily to 600 mg once daily for 10-11 days were well-tolerated and had no clinically relevant effects on cardiac repolarization. 12In phase 2 clinical studies, 9,13,14 a dose of 400 mg twice daily demonstrated a favorable benefit-risk profile.Further evaluation of the 400 mg twice-daily dose of rilzabrutinib is ongoing in a phase 3 trial in patients with ITP. 15 Hepatic metabolism is the major route of elimination for rilzabrutinib in humans (cytochrome P450 [CYP] 3A being the major metabolizing enzyme) 12 with minimal renal excretion (approximately 3% eliminated unchanged in urine). 16Rilzabrutinib undergoes extensive metabolism and thiocyanate and a hydrolyzed metabolite (Figure S1) were among the identified circulating metabolites.In vitro and animal studies showed that both of these metabolites were inactive.An experiment in Caco-2 cells showed that rilzabrutinib is also a P-glycoprotein (P-gp) substrate.
The present study shows data from a phase 1 trial in healthy participants that was conducted to evaluate the potential drug-drug interaction (DDI) when rilzabrutinib was administered orally with quinidine (an inhibitor of P-gp and CYP2D6, used as an index inhibitor of CYP2D6 in DDI studies) 17 or rifampin (an inducer of CYP3A and P-gp). 18

Study Participants
This phase 1 trial was a single-center, open-label, 2part study conducted at the Clinical Pharmacology of Miami Centre, Florida, United States.The study enrolled healthy males and nonpregnant, nonlactating females aged 18-65 years (inclusive), with body mass index ranging from 18.0 to 35.0 kg/m 2 (inclusive) who were not smoking, not vaping, and not using tobacco.Participants willing to abstain from consuming alcohol-containing beverages or food, grapefruit, star fruit, and Seville orange products were enrolled prior to baseline/screening (14 days prior to the first dose).All participants provided written informed consent before the initiation of the study.The study was conducted in accordance with the principles of the Declaration of Helsinki and in compliance with the International Council for Harmonization of Technical Requirements for Pharmaceuticals for Human Use E6 Guideline for Good Clinical Practice.The protocol was reviewed and approved by the IntegReview Institutional Review Board, Austin, Texas, United States.

Study Design
The primary objective of the study was to assess the impact of quinidine (300 mg) or rifampin (600 mg) coadministration on the pharmacokinetics (PK) of rilzabrutinib (400 mg).The secondary objective was to assess the safety and tolerability of rilzabrutinib when coadministered with quinidine or rifampin.
In this study, Part A evaluated the effect of quinidine on rilzabrutinib PK and Part B evaluated the effect of rifampin on rilzabrutinib PK.Each part was an openlabel, fixed-sequence, 2-period, 2-intervention study in healthy participants.In this study, separate participants were enrolled in Parts A and B. In both Parts A and B, participants first received a single dose of rilzabrutinib (Period 1), and then, after a washout period, they received multiple doses of either quinidine or rifampin over several days before another single dose of rilzabrutinib (Period 2).In Part A, in the morning of day 1, participants received a single oral dose of 400 mg of rilzabrutinib with 240 mL of water following an overnight fast (Period 1).These participants were scheduled to receive 300 mg of quinidine every 8 hours (Q8h) for 5 days (a total of 15 doses) from day 7 to day 11 (Period 2).In the morning of day 10 (following an overnight fast), a single oral dose of 400 mg of rilzabrutinib was administered approximately 1 hour after the first 300-mg quinidine dose.In Part B, participants received a single oral dose of 400 mg of rilzabrutinib with 240 mL of water following an overnight fast on day 1 morning (Period 1).The same participants were scheduled to receive 600 mg of rifampin once daily in the morning for 11 days (a total of 11 doses) from day 7 through day 17.In the morning of day 16 (following an overnight fast), a single oral dose of 400 mg of rilzabrutinib was co-administered with 600 mg of rifampin (Period 2).Plasma samples for the determination of concentration of rilzabrutinib and its metabolites (hydrolyzed metabolite and thiocyanate) and for the evaluation of quinidine PK were collected.

PK Assessments
In Part A, blood samples to determine rilzabrutinib plasma concentrations were obtained on Period 1 day 1 and Period 2 day 10 at 0 (pre-dose), 0.5, 1, 1.5, 2, 2.5, 3, 4, 6, 8, 12, 16, 24, 30, 36, and 48 hours postdose.Blood samples to determine quinidine plasma concentrations were obtained on Period 2 days 7 and 10 at 0 (pre-dose), 0.5, 0.75, 1, 1.5, 2, 3, 4, 6, and 8 hours postdose, and days 8 and 9 prior to each morning dose of quinidine.In Part B, blood samples to determine rilzabrutinib plasma concentrations were obtained on Period 1 day 1 and Period 2 day 16 at 0 (pre-dose), 0.5, 1, 1.5, 2, 2.5, 3, 4, 6, 8, 12, 16, and 24 hours postdose.As a part of assessing the impact of quinidine and rifampin on rilzabrutinib PK and clearance pathways, rilzabrutinib metabolites (hydrolyzed metabolite and thiocyanate) were also monitored.Sample values below the limit of quantification (BLQ) (<0.1 ng/mL for plasma rilzabrutinib and <100 ng/mL for plasma quinidine) were set to 0 for calculation of the mean and standard deviation (SD).If all samples at a time point were BLQ, mean and SD are not provided.If mean and SD were less than 0, then 0 is displayed for mean and SD.The treatment groups were offset for readability at hour 4 and later.

Analytical Methods
Rilzabrutinib and Hydrolyzed Metabolite.Plasma rilzabrutinib and its metabolite concentrations were measured by Alturas Analytics Inc. (Moscow, USA) using a fully validated high-performance liquid chromatography/tandem mass spectroscopy (LC-MS/MS) assay on a Sciex API 5500 triple quadrupole mass spectrometer (AB Sciex LLC, MA, USA) coupled with a Shimadzu LC system.Mobile phases of the LC separation were 1% formic acid in water (phase A) and 1% formic acid in acetonitrile (phase B), respectively.The respective multiple reaction monitoring (MRM) transitions were 666.5-524.4for rilzabrutinib and 472.2-322.1 for hydrolyzed metabolite with optimized collision energies of 35 and 30 V, respectively.The deuterated rilzabrutinib and the hydrolyzed metabolite were used as the internal standards.The lower limit of quantitation (LLOQ) for rilzabrutinib and hydrolyzed metabolite was 0.100 ng/mL, with a dynamic range of 0.100-100 ng/mL.Detailed methodology is presented in the Supplemental Information.
Thiocyanate.Thiocyanate concentrations in human K2EDTA plasma were also determined using the same LC-MS/MS system setup as described above.The LC separation was performed with mobile phase A of 4.9 mM ammonium acetate (pH 7) and mobile phase B of acetonitrile.The MRM transition of 248.1-111.3 was used for monitoring derivatized thiocyanate from the reaction with 5,5 -dithiobis(2-nitrobenzoic acid) (Ellman's reagent) with the protocol reported as previously. 19Isotopically labeled internal standard, SCN 13 C 15 N, was used for this analysis.The LLOQ for monitoring thiocyanate was 1000 ng/mL, with a calibration dynamic range of 1000-50,000 ng/mL.Detailed methodology is presented in the Supplemental Text (Analytical methods).
Quinidine.Quinidine concentrations in human K2EDTA plasma were determined using the Sciex API-4000 system coupled with a Shimadzu LC system (AB Sciex LLC, MA, USA).The LC separation was performed on a Supelco Discovery HS-C18 column (2.1 × 50 mm with 3 μm particles) with 1% formic acid in water (phase A) and 1% formic acid in acetonitrile (phase B), respectively.The MRM transition of 325.3-160.3 was used for the detection of quinidine.The internal standard was quinidine-d3 for this analysis.The LLOQ for quinidine was 100 ng/mL, with a calibration range of 100-10,000 ng/mL.Detailed methodology is presented in the Supplemental Text (Analytical methods).
In all analytical methods, the accuracy (% bias) for each standard point was within 15% (20% at the LLOQ) of the nominal value which was considered acceptable.

Safety
The safety and tolerability of the study treatments were assessed throughout the studies via physical examination, laboratory tests, vital signs, electrocardiograms (ECGs), and observation of adverse events (AEs), serious adverse events (SAEs), and treatment-emergent adverse events (TEAEs).AEs were coded using the Medical Dictionary for Regulatory Activities, Version 23.1.These AEs were summarized by part and treatment for the number of subjects reporting TEAEs and the number of TEAEs reported.

Statistical Analysis
In this study, 32 evaluable participants (16 for each part) were enrolled; in Part A, the PK analysis set consisted of 9 participants (7/16 discontinued early from the study), while in Part B, the PK analysis sets consisted of 16 participants (all participants completed the study).
All participants who were enrolled in the study and had received at least 1 dose of study drug were included in the safety analysis set.All participants who had sufficient plasma concentration data available to compute at least 1 PK parameter were included in the PK analysis set.The PK and safety analysis sets were determined separately for each period.
PK parameters were calculated from plasma rilzabrutinib and quinidine concentrations using standard noncompartmental methods using Phoenix WinNonlin (Certara, NJ, USA), which included maximum observed concentration (C max ), time of maximum observed concentration (T max ), area under the concentration-time curve from time 0 to the select time (AUC 0-t ), area under the concentration-time curve from time 0 to the last quantifiable concentration (AUC 0-last ), area under the concentration-time curve from time 0 extrapolated to infinity (AUC 0-∞ ), apparent total plasma clearance after oral (extravascular) administration (CL/F), and half-life (t 1 2 ). Summary statistics of plasma concentrations and PK parameters (primary and secondary) are presented for each treatment period, including means, geometric means, SD, coefficient of variation (CV), medians, and ranges, as appropriate.The individual and mean plasma concentration versus time data were plotted on linear and semilogarithmic scales.No value for AUC 0-∞ , CL/F, or t 1 2 was reported for cases that did not exhibit a terminal log-linear phase in the concentration-time profile.
To assess the effect of co-administration of quinidine and rifampin (perpetrators) on rilzabrutinib in plasma, the estimated means difference (perpetrator and rilzabrutinib [test] -rilzabrutinib alone [reference]) and its 90% confidence interval (CI) limits on the logscale were constructed.For each primary PK parameter (AUC 0-last , AUC 0-∞ , and C max ), an estimate of the adjusted geometric mean ratios (GMRs) for the comparison of interest (test/reference) was calculated by exponentiation of the difference between estimated least squares means.Similarly, the 90% CI of the estimated means difference was back-transformed to obtain the results on the original scale (ie, to calculate the 90% CI of the GMR).
Analysis of variance was applied to the logtransformed primary PK parameters.All summary statistics for rilzabrutinib and quinidine PK parameters were generated using SAS Version 9.4 (SAS Analytics Solutions, NC, USA) or above.

Study Participants
In this study, a total of 16 healthy participants were enrolled in Part A (assessment of interactions of rilzabrutinib with quinidine).Of these, 8 participants completed the study and 8 experienced a TEAE leading to discontinuation of the study, namely QT prolongation in the ECG occurring more than 7 days after the last rilzabrutinib administration and 1-3 days after start of quinidine administration.These ECG abnormalities were therefore consistent with the known effects of quinidine on QT and were not believed to be related to rilzabrutinib.Among the participants with QT prolongation, 7 discontinued the study before co-administration of rilzabrutinib and 1 participant discontinued the study after rilzabrutinib co-administration.No deaths or SAEs were reported.

Demographic and Baseline Characteristics
The demographic and baseline characteristics of the participants enrolled in this study are provided in Table S2.

Pharmacokinetic Assessments
Rilzabrutinib Co-administration with Quinidine.The plasma quinidine concentration-time profile following the initial quinidine dose was compared with the profile following co-administration of rilzabrutinib and quinidine after 5 days of quinidine dosing.In both situations, peak concentrations of quinidine occurred between 1 and 1.5 hours postdose but were higher at steady state, while the mean plasma concentrations appeared to decline at a similar rate (Figure 1).The mean predose and 8-hours postdose quinidine concentrations at day 10 differed by 9.5% (1228 vs 1345 ng/mL, respectively), indicating that at least 90% of steady-state conditions were obtained.A summary of PK parameters is provided in Table S3.
The plasma rilzabrutinib concentration-time profiles from 0 to 48 hours postdose after administration of a single 400-mg dose of rilzabrutinib alone and with co-administration with quinidine are presented as linear and semilog plots in Figure 2A,B.Based on visual inspection, the maximum plasma concentrations were attained quickly postdose for both treatments.The mean rilzabrutinib plasma concentrations after co-administration of rilzabrutinib 400 mg and quinidine were slightly lower at all time points than those after administration of rilzabrutinib 400 mg alone.The mean plasma concentrations appeared to decline at a similar rate for both treatments (Figure 2).A summary of PK parameters for rilzabrutinib 400 mg single dose alone or when co-administered with quinidine is presented in Table 1.Rilzabrutinib reached C max by 4 hours postdose in all participants, with individual C max values ranging from 23 to 386 ng/mL when administered alone and from 19 to 232 ng/mL when co-administered with quinidine (Table 1).Median T max for rilzabrutinib was similar between the 2 treatments.Although the t 1 2 of rilzabrutinib was shorter after co-administration with quinidine (8.2 hours) than after administration of rilzabrutinib alone (17.2 hours), the elimination rates, as observed by the log-linear decline in mean concentration-time profile, were similar up to 30 hours postdose, with only a slower rate in the most terminal portion of the profile (30-48 hours) than that after rilzabrutinib alone (Figure 2).The duration of the sampling scheme (0-48 hours postdose) was sufficient to robustly characterize the exposure to rilzabrutinib for both treatments, with an average of 2% or lower of the AUC extrapolated to infinity, compared with the AUC 0-last .The mean apparent systemic clearance (CL/F) for rilzabrutinib was slightly higher (17.2%) after co-administration of rilzabrutinib with quinidine (CL/F 1360 L/h) than that after administration of rilzabrutinib alone (CL/F 1160 L/h) due to a slight decrease in systemic exposure (AUC 0-∞ ) of rilzabrutinib.

(A) (B)
Figure 1.Mean (+ standard deviation) plasma quinidine versus time profiles following the initial quinidine dose and following co-administration of rilzabrutinib and quinidine after 5 days of quinidine dosing (rilzabrutinib + quinidine).Data (pharmacokinetics analysis set) are graphed using (A) linear scale and (B) semi-log scale.Some symbols representing the mean were offset relative to time for readability.
A summary of statistical comparisons of rilzabrutinib PK parameters when administered alone or when co-administered with quinidine (Part A) is presented in Table 2.No significant change was observed in rilzabrutinib PK in the presence of quinidine.The 90% CI of all GMRs for AUCs and C max of rilzabrutinib included equivalence (GMR = 1) and the CI ranges were wide due to the high within-subject variability, especially relative to the sample size of the study.No significant difference was observed in AUC 0-∞ and C max of hydrolyzed metabolite between rilzabrutinib and rilzabrutinib plus quinidine groups (Table S4).
The plasma levels of thiocyanate during the quinidine treatment period were generally lower than those observed during the rilzabrutinib alone period, possibly due to the variation of endogenous level of thiocyanate.Some participants in both treatment groups had no (A) (B) measurable plasma concentrations of thiocyanate, limiting the interpretation of data (Table S5).
Rilzabrutinib Co-administration with Rifampin.The plasma rilzabrutinib concentration-time profiles from 0 to 24 hours postdose after administration of a single 400-mg dose of rilzabrutinib alone and with co-administration with rifampin are presented as linear and semilog scales in Figure 3A,B.The mean rilzabru-tinib plasma concentrations increased over the first 2.5 hours, with peak concentrations observed around 2.0 hours postdose for both treatments (Figure 3A).The mean rilzabrutinib plasma concentrations after co-administration of rilzabrutinib and rifampin were substantially lower at all time points than those observed after administration of rilzabrutinib alone.The mean plasma concentrations of rilzabrutinib AUC 0-24 , area under the concentration-time curve from time 0 to the select time (t), as calculated by the linear trapezoidal method; AUC 0-∞ , area under the concentration-time curve from time 0 extrapolated to infinity; AUC 0-last , area under the concentration-time curve from time 0 to the last quantifiable concentration; CL/F, apparent total plasma clearance after oral (extravascular) administration, calculated as dose/AUC 0-∞ (parent only); C max , maximum observed concentration; CV, coefficient of variation; n, number of participants with valid observations; N, number of participants; NA, not available; PK, pharmacokinetics; SD, standard deviation; t 1/2 , terminal elimination half-life; T max , time of maximum observed concentration.a n = 12.
b n = 8. c n = 14.ANOVA, analysis of variance; AUC 0-∞ , area under the concentration-time curve from time 0 extrapolated to infinity; AUC 0-last , area under the concentration-time curve from time 0 to the last quantifiable concentration; CI, confidence interval; C max , maximum observed concentration; GLSM, geometric least-squares mean; n, number of participants with valid observations; PK, pharmacokinetics.
a From an ANOVA model for the log-transformed parameter results with fixed effect treatment and random effect subject.
appeared to decline at a similar rate for both treatments, as observed by the log-linear decline in the mean concentration-time profile (Figure 3B).A summary of PK parameters for rilzabrutinib 400 mg when administered alone or in combination with rifampin 600 mg is presented in Table 1.Rifampin treatment substantially decreased the plasma exposure of rilzabrutinib, reflecting its metabolic CYP3A4 induction effects.The mean t 1/2 of rilzabrutinib decreased from 4.6 to 2.9 hours when co-administered with rifampin compared with rilzabrutinib alone.The CL/F was substantially increased from the rilzabrutinib-only (A) (B) Figure 3. Mean (+ standard deviation) plasma rilzabrutinib concentration versus time profiles after administration of a single 400 mg dose of rilzabrutinib only and after co-administration with rifampin in healthy adult participants.Data (pharmacokinetics analysis set) are graphed using (A) linear scale and (B) semi-log scale.Some symbols representing the mean were offset relative to time for readability.
group in the co-administered rilzabrutinib and rifampin group, by ∼3-fold, reflecting the metabolic induction effect of rifampin by decreasing the AUC 0-∞ and increasing elimination rate (K el ) of rilzabrutinib.Rifampin appears not to have affected the rate of absorption of rilzabrutinib, since the median T max in both treatments was 2.0 hours.
A summary of statistical comparisons of rilzabrutinib PK parameters after administration of rilzabrutinib 400 mg alone or after co-administration of rilzabrutinib 400 mg and rifampin (Part B) is presented in Table 2.After co-administration of rilzabrutinib 400 mg and rifampin, C max , AUC 0-∞ , and AUC 0-last of rilzabrutinib were decreased by approximately 80.5%, 79.5%, and 80.1%, respectively, from that after rilzabrutinibonly, the geometric mean of C max and AUC 0-∞ for hydrolyzed metabolite decreased by 69.7% and 72.4%, respectively (Table S4), while the effect of rifampin on the disposition of thiocyanate could not be evaluated as most participants had no measurable plasma concentrations of thiocyanate (Table S5).

Safety
In Part A, the most frequently reported TEAEs included prolongation of the QT interval in the ECG (8 of 16 participants [50.0%]: 7 participants from the quinidine group and 1 participant from the rilzabrutinib and quinidine group), diarrhea (4 of 16 participants [25.0%]: 3 participants from rilzabrutinib group and 1 participant from the rilzabrutinib and quinidine group), and palpitations (4 of 16 participants [25.0%]: all 4 participants from the quinidine group) (Table S6).All 8 participants with ECG findings of prolonged corrected QT interval (QTc) were discontinued from the study.All TEAEs of prolonged QTc interval were reported as mild/grade 1, nonserious, and resolved without sequalae, and were considered related to quinidine use (none were reported as being related to rilzabrutinib).
In Part B, after co-administration of rilzabrutinib with rifampin, the most frequently reported TEAEs included chromaturia (6 of 16 participants, 37.5%) and diarrhea (3 of 16 participants, 18.8%) (Table S7).All TEAEs were reported as mild/grade 1, resolved, and nonserious.No TEAEs were reported that led to study drug discontinuation (drug withdrawn) or to discontinuation of the study.
No deaths or SAEs were reported when rilzabrutinib was co-administered with rifampin and quinidine.Overall, no clinically important observations were noted laboratory test results, vital signs, or ECG evaluation.

Discussion
The present analysis assessed the impact of multiple doses of quinidine (an inhibitor of P-gp and CYP2D6) or rifampin (an inducer of CYP3A and P-gp) on the PK of rilzabrutinib in healthy adult participants.
Quinidine exposure reached steady state within 2-3 days, and exposure levels of quinidine following the single dose and during the steady-state dosing were comparable, on a dose basis, to what has been described in the literature. 20Co-administration of quinidine with rilzabrutinib had no clinically meaningful effect on rilzabrutinib AUCs or C max .The median elimination t 1/2 of rilzabrutinib was shorter after quinidine coadministration than after rilzabrutinib only (4.5 vs 14.2 hours).However, in previous studies, 10,12 the t 1/2 of rilzabrutinib was less than 4 hours.Therefore, the high value for t 1/2 a following rilzabrutinib only in the current study may be due to a high variability in t 1/2 .
The most frequently reported TEAEs included prolonged QTc interval (50.0%) during the quinidine dosing phase.All 8 participants with QTc prolongation (related to quinidine) were discontinued from the study.Despite the early discontinuation of the quinidine administration in the participants who received quinidine and rilzabrutinib, they had sufficient plasma concentration to compute at least 1 PK parameter that helped in evaluating the effect of quinidine on the PK of rilzabrutinib in healthy adult participants.All TEAEs of QT prolongation were consistent with previously reported AEs associated with quinidine usage. 21,22o-administration of rifampin decreased rilzabrutinib exposure compared with rilzabrutinib alone.The mean elimination t 1/2 of rilzabrutinib was shorter when co-administered with rifampin.The lower exposure and shorter t 1/2 observed for rilzabrutinib after 11 days dosing with rifampin indicated a major role for CYP3A in the metabolism of rilzabrutinib.This confirmed the conclusion from a previously published study of the effects of ritonavir, a strong inhibitor of CYP3A 23 and an inhibitor of P-gp, on the pharmacokinetics of rilzabrutinib. 12Specifically, ritonavir co-administration increased rilzabrutinib C max by 5.2-fold and AUC 0-∞ by 8.3-fold.
Any contribution of P-gp to the clearance of rilzabrutinib is not clear from DDI studies using rifampin (the current study) and ritonavir. 12However, the lack of an effect of quinidine on rilzabrutinib exposure in the current study rules out a significant role of P-gp in the pharmacokinetics of rilzabrutinib.In addition, these results rule out a significant role of CYP2D6 in the clearance of rilzabrutinib.
The concentration-time course of the hydrolyzed metabolite (with quinidine co-administration), for the most part, closely followed that of rilzabrutinib.Quinidine, an inhibitor of CYP2D6, 24 had no relevant effect on the disposition of the hydrolyzed metabolite.On the other hand, rifampin treatment resulted in decreases in C max and AUC 0-∞ , respectively, of the hydrolyzed metabolite, similar to the decreases observed with rilzabrutinib.The substantial decreases seen in hydrolyzed metabolite exposure with rifampin treatment suggest that its formation from rilzabrutinib was not mediated by CYP3A4.Alternatively, its metabolic elimination could be mediated by CYP3A4 as observed in vitro. 25he mean AUC 0-∞ ratios of hydrolyzed metabolite to that of rilzabrutinib in Period 1 of Parts A and B (rilzabrutinib alone) were 1.0 (Part A) and 0.9 (Part B).These ratios increased following quinidine and rifampin treatment to 1.5 and 1.1, respectively.The significance of this change is considered minimal, given it is an inactive metabolite, and its PK variability was large (CV% 40.2% [Part A] and 38.5% [Part B]).It is acknowledged that the mechanism of metabolite clearance and how the perpetrator affects the metabolite clearance are not fully understood.
The rilzabrutinib metabolite thiocyanate is an endogenous compound mainly derived from dietary sources.In the study population, the plasma levels of thiocyanate both at baseline and throughout the time course of the studies were highly variable, consistent with the reported literature for the general population. 26Furthermore, in Part A, only 11 of the 16 participants in Period 1 and 4 of the 9 participants in Period 2 had quantifiable plasma thiocyanate concentrations.In Part B, only 3 of the 16 participants in Period 1 and none of the 16 participants in Period 2 had measurable plasma concentrations of thiocyanate.The high degree of variability made it difficult to assess whether thiocyanate in the plasma postdose was derived from rilzabrutinib.While rilzabrutinib may contribute to plasma thiocyanate levels, interindividual variability of thiocyanate concentrations, likely in part due to differences in diet, rendered these results inconclusive.

Conclusion
Co-administration of rilzabrutinib with quinidine resulted in similar rilzabrutinib exposure as observed with rilzabrutinib alone.This rules out a significant role of P-gp mediated enteric efflux in the pharmacokinetics of rilzabrutinib.Co-administration of rilzabrutinib with rifampin resulted in a substantial decrease in exposure to rilzabrutinib.This, in turn, supports the previously published conclusion that CYP3A is the principal metabolizing enzyme for rilzabrutinib, based on a large increase in exposure to rilzabrutinib after ritonavir coadministration. 12The PK characteristics of rilzabrutinib elucidated in this study may help to better understand its DDI potential.
Part A were enrolled into Part B. A total of 16 healthy participants were enrolled in Part B (assessment of interactions of rilzabrutinib with rifampin) of the study.All 16 participants completed Part B of the study.

Figure 2 .
Figure 2. Mean plasma rilzabrutinib concentration versus time profiles after administration of a single 400 mg dose of rilzabrutinib only and after co-administration with quinidine in healthy adult participants.Data (pharmacokinetics analysis set) are graphed using (A) linear scale and (B) semi-log scale.Error bars were omitted for readability and some symbols representing the mean were offset relative to time for readability.

Table 1 .
Summary of Plasma Rilzabrutinib PK Parameters with and without Quinidine and Rifampin (PK Analysis Set)

Table 2 .
Statistical Analysis of the Effect of Quinidine and Rifampin Co-administration on Rilzabrutinib PK All the values in the table are GLSM values.