British Journal of ClinicalPharmacology

DOI:10.1111/bcp.12584

© 2

Pharmacokinetic interactionsamong imatinib, bosentanand sildenafil, and theirclinical implications in severepulmonary arterialhypertensionDidier Renard,1 Thomas Bouillon,1 Ping Zhou,2 Gerard Flesch1

& Debbie Quinn3

1Novartis Pharma AG, Basel, Switzerland 2Novartis Horsham Research Centre, Horsham, West

Sussex, UK and 3Novartis Pharmaceuticals, East Hanover, USA

WHAT IS ALREADY KNOWN ABOUTTHIS SUBJECT

characterized in patients with chronicmyeloid leukemia (CML) andgastrointestinal stromal tumours (GIST).

• Imatinib improved exercise capacity and

PAH who remain symptomatic on at leasttwo drugs of the currently available drugclasses.

• PK interactions between bosentan and

WHAT THIS STUDY ADDS• Population PK of imatinib in severe PAHwere comparable with CML.

• Bosentan and sildenafil concentrationswere elevated on co-administration withimatinib.

• Imatinib has intrinsic efficacy beyond

bosentan and sildenafil.• There was no evidence of increased liver-related toxicity with co-administration ofbosentan and imatinib.

015 The British Pharmacological Society Br J Clin Ph

CorrespondenceDr Didier Renard, Integrated QuantitativeSciences - Pharmacometrics, NovartisPharma AG, Postfach, CH-4002 Basel,Switzerland.Tel: +416 1324 1863Fax: +41 61 3241246E-mail: didier.renard@novartis.com----------------------------------------------------

Keywordsbosentan, drug–drug interactions,imatinib, pharmacokinetics, pulmonaryarterial hypertension, sildenafil----------------------------------------------------

Received13 May 2014

Accepted31 December 2014

Accepted ArticlePublished Online7 January 2015

• Population PK of imatinib have been

AIMSThis study characterized the population pharmacokinetics (PK) ofimatinib in patients with severe pulmonary arterial hypertension (PAH),investigated drug–drug interactions (DDI) among imatinib, sildenafiland bosentan, and evaluated their clinical implications.

haemodynamics in patients with advanced

METHODSPlasma concentrations of imatinib, bosentan and sildenafil were collectedin a phase III study and were used to characterize the PK of imatinib in thispopulation. DDIs among the three drugs were quantified using a linearmixed model and log-transformed drug concentrations.

sildenafil have been reported.

RESULTSThe population mean estimates of apparent clearance (CL/F) and volume(V/F) were 10.8 l h–1 (95% CI 9.2, 12.4 l h–1) and 267 l (95% CI 208, 326 l),respectively. It was estimated that sildenafil concentrations increased, onaverage, by 64% (95% CI 32%, 103%) and bosentan concentrations by51% (95% CI 12%, 104%), in the presence of imatinib. Despite increasedconcentrations of co-medications, treatment differences betweenimatinib and placebo for change in 6 min walk distance and pulmonaryvascular resistance were relatively constant across the entire concentrationrange for sildenafil and bosentan. Overall, higher concentrations of imatiniband bosentan were not associated with increasing liver enzymes (serumglutamic oxaloacetic transaminases [SGOT]/serum glutamic-pyruvictransaminase [SGPT]).

merely elevating plasma concentrations of

CONCLUSIONSPopulation PKs of imatinib in patients with severe PAH were foundcomparable with those of patients with chronic myeloid leukemia.Imatinib was found effective regardless of the co-medications andshowed intrinsic efficacy beyond merely elevating the concentrationsof the co-medications due to DDIs. There was no evidence of increasedrisk of liver toxicity upon co-administration with bosentan.

armacol / 80:1 / 75–85 / 75

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Introduction study included evaluation of safety and efficacy ofimatinib in patients with PAH. One of the secondary

Pulmonary arterial hypertension (PAH) is a progressivedisease with poor prognosis, characterized by markedand sustained elevation of pulmonary arterial pressure(PAP), pulmonary vascular resistance (PVR) and incre-mental pulmonary vasculopathy that ultimately leads topremature death [1–4]. The endothelin, nitric oxide andprostacyclin pathways are three physiological pathwaysthat play an important role in the pathophysiology ofPAH [5]. These pathways are primarily associated withvasodilation and unregulated proliferation of pulmonaryartery vascular smooth muscle cells [2].

Currently three classes of drugs have been approved forthe treatment of PAH in Europe and the United States,namely, prostacyclin analogues, endothelin receptor antag-onists (ambrisentan, bosentan, tadalafil and epoprostenol)and phosphodiesterase (PDE5) inhibitors (avanafil, sildena-fil, lodenafil, mirodenafil, tadalafil, vardenafil, udenafil andzaprinast). There is limited improvement in pulmonaryhaemodynamics as the current therapeutic interventionsavailable, including bosentan and sildenafil, primarily tar-get pulmonary vasodilatation, whereas PAH is a prolifera-tive disease of small pulmonary resistance vessels. Thus,mortality remains high among patients with PAH. The me-dian survival of patients with idiopathic or heritable PAH is<3years [6], despite current therapy, which highlights theneed for more treatment options [7–9].

Imatinib is a tyrosine kinase inhibitor (TKI) targetingthe Abelson tyrosine kinase (ABL1), together withthe Abelson-related kinase (ABL2) and the oncogenicBCR-ABL fusion protein, platelet-derived growth factorreceptor (PDGFR)-α and β, discoidin domain receptor(DDR) and the KIT receptor [4]. Imatinib is approvedfor the treatment of various malignant disorders includ-ing Philadelphia (Ph) chromosome-positive chronic my-eloid leukemia (CML), acute lymphoblastic leukemiaand gastrointestinal stromal tumours (GIST).

Imatinib has been shown to inhibit certain cytochromeP450 (CYP450)-metabolizing enzymes, and thus drug–druginteractions (DDIs) may occur. Imatinib is a substrate forCYP3A4/5 and has been shown, in vitro, to be a competitiveinhibitor of CYP3A4/5, CYP2C9 and CYP2D6 [4]. Drugs thatinhibit or induce the CYP3A4 isozyme have been shown toalter imatinib pharmacokinetic (PK) exposure [10]. Sildenafilis metabolized predominantly by CYP3A4 and to a minorextent by CYP2C9 [11]. Bosentan is metabolized in the liverby CYP3A4 and CYP2C9 [12, 13]. Mutual PK interactionsbetween bosentan and sildenafil have been reported inhealthy volunteers, and the dosage of each drug in a com-bination treatment may have to be adjusted accordingly.Thus, reciprocal PK interactions on co-administration ofthese three drugs warranted investigation.

IMPRES [14], a multicentre, randomized, double-blind,placebo-controlled, 24 week trial, evaluated imatinib inpatients with severe PAH. The primary objective of this

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objectives of this study was to assess the PK of imatinibin this patient population and the potential for interac-tion of imatinib on sildenafil and bosentan.

The present analysis was performed primarily tocharacterize the population PK of imatinib in severePAH and to determine the PK interactions amongimatinib, bosentan and sildenafil. After this analysis con-firmed the presence of significant interactions amongthe three drugs, the effect of the intrinsic efficacy of ima-tinib, as well as the potential for increased risk of hepa-totoxicity when it is co-administered with bosentan,was assessed.

Methods

Study participantsThe study population consisted of 202 adult males andfemales aged ≥18years, with a diagnosis of severe PAH, de-fined as those who remained symptomatic, i.e. had WHOfunctional class II-IV status, were on at least two PAH-specifictherapies and had a baseline PVR of ≥800dyn s cm-5. PAHcould be either idiopathic or heritable (familial or sporadic).It could be associated with (a) collagen vascular disease in-cluding systemic sclerosis, rheumatoid arthritis, mixed con-nective tissue diseases and overlap syndrome, (b) the use ofappetite suppressants or toxic compounds or (c) congenitalheart disease (≥1 year post-complete repair of atrial septal de-fect, ventricular septal defect or posterior descending artery).

Study design and treatmentsThis was a multicentre, randomized, double-blind andparallel group study in patients with PAH. After informedconsent was obtained, the patients were screened toevaluate the pulmonary haemodynamics, and if foundsuitable, were randomized in 1: 1 ratio to receive imatinib(in 100mg film-coated tablets) or placebo once daily. Thetherapy was initiated with 200mg imatinib for 2weeks,followed by 400mg imatinib, if tolerated well, until24weeks. If 400mg of imatinib was not tolerated, thedose was down-titrated to 200mg. Details of the studydesign and main results are described elsewhere [14].The study protocol was approved by ethics committeesand/or institutional review boards at each study centreand each patient provided written, informed consent toparticipate in the study.

PK assessments and monitoringA sparse PK sampling approach was taken and plasmasamples were typically obtained at the following timesin the study: day 0 (first 200mg once daily dose, at pre-dose and between 0.5 and 3 h post-dose), day 14 (first400mg once daily dose, at pre-dose and between 0.5and 3 h post-dose), day 28 (at pre-dose and between

PK analysis of imatinib, bosentan and sildenafil in patients with PAH

0.5 and 3 h post-dose) and day 168 (at pre-dose, between0.5 and 3 h post-dose, between 3 and 6 h post-dose andbetween 6 and 8 h post-dose).

All samples were taken by either direct venipunctureor via indwelling cannula inserted in a forearm vein. Foreach plasma sample, 6ml of blood were collected into atube containing heparin, inverted several times and cen-trifuged at 1100 g for at least 10min. Plasma sampleswere separated into polypropylene screw-cap tubesand frozen at –20 °C. All tubes were kept frozen untilshipment. All samples were carefully packed in suitablepacking material containing sufficient dry ice to keepthem frozen during shipment.

The parent compound imatinib and its metabolite,CGP74588, were measured in plasma by validated liquidchromatography-mass spectrometry (HPLC-MS/MS) assay[15]. The limit of quantification for imatinib and its activemetabolite assays was 20ng ml–1. The parent drugbosentan and its major active metabolite, Ro 48-5033, weredetermined by validated HPLC-MS/MS assays. The limit ofquantification for bosentan and its active metabolite was1ng ml–1. The parent drug sildenafil and its active N-desmethyl metabolite were determined by validatedHPLC-MS/MS assays. The limit of quantification for sildenafiland its active metabolite was 1ng ml–1.

Statistical methods

Population PK analysisThe population PK of imatinib was described by a onecompartment disposition model with zero order inputand inter-individual variability (IIV) on CL/F and volumeof distribution (V/F). The covariate search included age,gender, race, haemoglobin, white blood cell (WBC) countand co-medications (CYP3A4 inhibitors such as sildenafiland bosentan). Two covariates, presence/absence ofbosentan and haemoglobin concentrations, wereincluded in the final model. More details on methodsfor covariate searching and validation of the populationPK model are separately provided as an online appendix.Population PK analyses were performed with NONMEM(version VI, Icon Development Solutions).

Drug–drug interaction assessmentFor graphical exploration, dose-normalized concentrations ofone drug were plotted vs. absolute concentrations of thesecond drug in the presence or absence of the third drug.Only concentrations at steady-state for all three drugs wereincluded, thus excluding day 0 (i.e. first dose of imatinib).

To quantify interaction effects more precisely, con-centrations of sildenafil (respectively, bosentan) werelog transformed and analyzed using a linear mixedmodel that included total daily dose and baselineconcentration of sildenafil (respectively, bosentan) ascontinuous covariates, and indicator variables for the

presence/absence of bosentan (respectively, sildenafil)and imatinib. Drug concentrations of sildenafil andbosentan at baseline were calculated as the average ofthe two concentrations obtained at day 0. The modelfurther included subject as a random effect. Geometricmean ratios with 95% confidence intervals (CI) were de-rived to quantify the mean fold difference in sildenafilconcentrations in the presence vs. absence of bosentanor imatinib. A similar approach was used to investigatethe effects of bosentan and sildenafil on imatinib con-centrations, with the analysis model including imatinibdose (log transformed) as a covariate and indicator vari-ables for the presence/absence of bosentan and sildena-fil. Those analyses were performed in R version 2.10.1using the LME function (NLME library).

The results were retrospectively contrasted withthose from a dedicated DDI study (unpublished data,NCT01392469; http://clinicaltrials.gov/show/NCT01392469),which was conducted at the request of health authori-ties. This DDI study focused on characterizing the PKeffect of imatinib on the co-administered drugs bosentanand sildenafil. Changes in exposure (AUC over dosinginterval) of sildenafil and bosentan, before and afteradministration of imatinib (200mg for 2weeks followedby 400mg for 2weeks), were used for this purpose.

Clinical implications of drug–drug interactionfindingsGraphical exploration to investigate relationships betweenplasma concentrations of each drug to key efficacy andsafety variables was undertaken.

Results

Two hundred and two adult patients with severe PAH wererandomized to receive either imatinib (n=103) or placebo(n=99). In total, 69 patients (67%) in the imatinib treatmentarm and 81 (81.8%) patients in the placebo treatment armcompleted the study. We refer to the original publication[14] for additional details related to the study population.

The overall PK analysis dataset consisted of 751measur-able concentrations of bosentan, 1024 of sildenafil and 572of imatinib collected from 191 PAH patients. Among these,101 patients received at least one dose of imatinib, 165received sildenafil and 114 received bosentan during thestudy. Details of imatinib-treated patients who receivedsildenafil and bosentan are presented in Table 1.

Population PKs of imatinibThe population PK dataset of imatinib is represented inFigure 1, where dose-normalized concentrations at steady-state are plotted vs. time after last dose administration.Superimposed on this plot is a historical prediction fromthe population PK model of imatinib in CML [16]. As it canbe seen, the curve and corresponding 90% prediction

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Table 1Number of patients per drug combination in population PK dataset.

Placebo (n = 90) Imatinib (n = 101)

Bosentan Bosentan

Sildenafil No Yes Sildenafil No Yes

No 0 10 No 5 11

Yes 36 44 Yes 36 49

Figure 1Comparison of dose-normalized imatinib concentrations in PAH withhistorical prediction in CML. Dose-normalized concentrations mea-sured in the study (circles) were overlaid with a historical populationprediction (solid line, population median; coloured area, 90% predic-tion interval) from the CML population model of imatinib.

Table 2Parameter estimates of the final population pharmacokinetic model forimatinib

Parameter Estimate (standard error)

CL/F (L/h) 10.8 (0.83) IIV: CV = 43%

V/F (L) 267 (30.0) IIV: CV = 64%

Fractional increase of CL/F and V/F dueto bosentan

0.46 (0.15)

Effect (power coefficient, b) of haemoglobinon V/F and CL/F, i.e. (Hb/128)

bwith Hb in g/L

0.49 (0.25)

Duration of first order input (h) 1.52 (0.15)

CL/F, apparent clearance of drug from plasma; CV, coefficient of variation; Hb,haemoglobin; IIV, inter-individual variability; V/F, apparent volume of distribu-tion at steady-state

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interval provide a reasonable description of the measuredconcentrations of imatinib in PAH, suggesting that PKs inthese two different patient populations are quite similar.

This was further confirmed by fitting the same struc-tural compartmental PK model (one compartment dis-position model with zero order absorption) to the PAHdataset. Parameter estimates of the final model for ima-tinib are shown in Table 2. The apparent clearance

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(10.8 l h–1, 95% CI 9.2, 12.4 l h–1) in the absence ofbosentan was similar to values previously reported inCML (13.8 ± 0.5 l h–1) and GIST (9.3 ± 1 l h–1) patients[16, 17]. The V/F (267 l, 95% CI 208, 326 l) was similar tothat in CML patients (252± 8 l) and approximately 45%greater than in GIST patients (184± 14 l). Bosentan wasestimated to increase apparent imatinib clearance andV/F by 46%, corresponding to a decreased exposure(AUC) of approximately 30%.

Drug–drug interactionsSildenafil concentrations tended to be reduced onco-administration with bosentan and increased withimatinib (Figure 2). The statistical analysis (Figure 3)estimated that sildenafil concentrations, on average,increased by 64% (95% CI 32%, 103%) in the presenceof imatinib and decreased by 44% (95% CI 30%, 56%) inthe presence of bosentan. The estimated combinedeffect of bosentan and imatinib was null (ratio relativeto no drug co-administered= 0.92, 95% CI 0.68, 1.23).Figure 3 also shows the estimated increase in sildenafilexposure (AUC) after administration of imatinib (redtriangle), as determined in the dedicated DDI study. Thiseffect was consistent with the estimate from our ownanalysis.

Increased bosentan concentrations were observedon co-administration of bosentan with sildenafil orimatinib (Figure 4). The statistical analysis (Figure 5)estimated that bosentan concentrations, on average,increased by 51% (95% CI 12%, 104%) in thepresence of imatinib and by 53% (95% CI 9%, 115%)in the presence of sildenafil. The estimated combinedeffect of sildenafil and imatinib was an increase inbosentan concentrations of 132% (95% CI 46%, 269%).Figure 5 also shows the estimated increase inbosentan exposure (AUC) after administration of ima-tinib (red triangle), as determined in the dedicatedDDI study. This effect was consistent with the estimatefrom our own analysis.

On co-administration with bosentan, imatinibconcentrations tended to decrease (Figure 6). In addition,there were no clear changes observed in imatinibconcentration on co-administration with sildenafil. Thestatistical analysis (Figure 7) confirmed that imatinib con-centrations, on average, decreased by 33% (95% CI 18%,45%) in the presence of bosentan and did not change ina statistically significant manner in the presence of sil-denafil (ratio present : absent = 0.96, 95% CI 0.76, 1.22).The estimated combined effect of sildenafil andbosentan was a decrease in imatinib concentrations of35% (95% CI 10%, 53%).

Clinical implications of DDI findingsThe analysis data set consisted of 186 patients (imatinibn=94, placebo n=92), which included measures ofefficacy (6 min walk distance (6MWD) and PVR) as well

Figure 2Effects of imatinib and bosentan on sildenafil concentrations. A, B: Log–log plot of dose-normalized sildenafil concentrations vs. (absolute) bosentan concen-trations conditioned by absence (A)/presence (B) of imatinib. Themedian is indicated by a line summarizing dose-normalized concentrations of sildenafil in theabsence (left cluster) or over a range of concentrations (right cluster) of bosentan. Bos, bosentan. C, D: Log–log plot of dose-normalized sildenafil concentrationsvs. (absolute) imatinib concentrations conditioned by absence (C)/presence (D) of bosentan. The median is indicated by a line summarizing dose-normalizedconcentrations of sildenafil in the absence (left cluster) or over a range of concentrations (right cluster) of imatinib. Ima, imatinib.

Figure 3Estimated effects of imatinib and bosentan on sildenafil concentrations. Geometric mean of sildenafil concentrations with 95% confidence intervals inthe presence/absence of imatinib and/or bosentan (A) and mean relative effects on sildenafil concentrations for co-administration of imatinib and/orbosentan vs. no co-administration (B). None, neither imatinib nor bosentan co-administered with sildenafil; Ima alone, imatinib co-administered; Bosalone, bosentan co-administered; Ima + Bos, imatinib and bosentan co-administered.◂, DDI study (geometric mean ratio for AUC over dosing interval)

PK analysis of imatinib, bosentan and sildenafil in patients with PAH

as safety indicators (liver enzymes, serum glutamicoxaloacetic transaminase [SGOT] and serum glutamic-pyruvic transaminase [SGPT]).

To assess the potential impact of increased exposureof co-medications on efficacy, 6MWD and PVR % changesfrom baseline, evaluated after 24weeks of treatment,

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Figure 4Effects of imatinib and sildenafil on bosentan concentrations. A, B: Log–log plot of dose-normalized bosentan concentrations vs. (absolute) sildenafil concen-trations conditioned by absence (A)/presence (B) of imatinib. Themedian is indicated by a line summarizing dose-normalized concentrations of bosentan in theabsence (left cluster) or over a range of concentrations (right cluster) of sildenafil. Sil, sildenafil. C, D: Log–log plot of dose-normalized bosentan concentrationsvs. (absolute) imatinib concentrations conditioned by absence (C)/presence (D) of sildenafil. The median is indicated by a line summarizing dose-normalizedconcentrations of bosentan in the absence (left cluster) or over a range of concentrations (right cluster) of imatinib. Ima, imatinib.

Figure 5Estimated effects of imatinib and sildenafil on bosentan concentrations. Geometric mean of bosentan concentrations with 95% confidenceintervals in the presence/absence of imatinib and/or sildenafil (A) and mean relative effects on bosentan concentrations for co-administrationof imatinib and/or sildenafil vs. no co-administration (B). None, neither imatinib nor sildenafil co-administered with bosentan; Ima alone, imatinibco-administered; Sil alone, sildenafil co-administered; Ima + Sil, imatinib and sildenafil co-administered.◂, DDI study (geometric mean ratio forAUC over dosing interval)

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Figure 6Effects of bosentan and sildenafil on imatinib concentrations. A, B: Log–log plot of dose-normalized imatinib concentrations vs. (absolute) sildenafil concentra-tions conditioned by absence (A)/presence (B) of bosentan. The median is indicated by a line summarizing dose-normalized concentrations of imatinib in theabsence (left cluster) or over a range of concentrations (right cluster) of sildenafil. Sil, sildenafil. C,D: Log–log plot of dose-normalized imatinib concentrations vs.(absolute) bosentan concentrations conditioned by absence (C)/presence (D) of sildenafil. The median is indicated by a line summarizing dose-normalizedconcentrations of imatinib in the absence (left cluster) or over a range of concentrations (right cluster) of bosentan. Bos, bosentan

Figure 7Estimated effects of bosentan and sildenafil on imatinib concentrations. Geometric mean of imatinib concentrations with 95% confidence intervals inthe presence/absence of bosentan and/or sildenafil (A) and mean relative effects on imatinib concentrations for co-administration of bosentan and/orsildenafil vs. no co-administration (B). None: neither bosentan nor sildenafil co-administered with imatinib; Bos alone, bosentan co-administered; Silalone, sildenafil co-administered; Bos + Sil, bosentan and sildenafil co-administered

PK analysis of imatinib, bosentan and sildenafil in patients with PAH

were plotted against individually averaged concentra-tions of sildenafil and bosentan (Figure 8).

Interpretation of those graphs requires caution asthey do not show typical concentration–response

relationships, as patients entered the study with someco-medications already prescribed by their treatingphysician. The interest of such displays primarily liesin contrasting the placebo and imatinib responses over

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Figure 8Relationship of 6MWD and PVR per cent change from baseline, evaluated after 24 weeks of treatment, vs. averaged sildenafil (left) and bosentan (right)concentrations. Percent changes from baseline in 6MWD (A) or PVR (B) after 24 weeks of treatment are plotted against the individually averaged silden-afil (left) and bosentan (right) concentrations. Averaged concentrations were obtained as the geometric mean of all measurable plasma concentrationsof sildenafil or bosentan in each patient. Circles are for patients in the imatinib group and triangles for patients in the placebo group. Patients not re-ceiving sildenafil (No Sil) or not receiving bosentan (No Bos) were assigned small random values for appearance on the logarithmic axes. In each plot, thedashed line corresponds to a smooth (loess) fit to the placebo data and the solid blue line to the imatinib data. The horizontal boxplots (in the lower partof each figure) refer to the distributions of the individually averaged sildenafil/bosentan concentrations in the placebo (Pbo) or imatinib (Ima) groups. Ineach boxplot, the bold line is the median value, the edges of the box correspond to the 1st and 3rd quartiles (hence length of the box = inter-quartilerange), and the whiskers extend to the most extreme data point that is not more than 1.5 times the inter-quartile range from the box

D. Renard et al.

the range of co-medication concentrations. A key pointto emphasise is that the treatment difference variesover the range of concentrations for sildenafil andbosentan. If efficacy of imatinib was partly attributableto increased exposure to sildenafil or bosentan, onewould expect to see increasing differences betweenplacebo and imatinib with higher concentrations ofthe respective co-medications. However, as shown in

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Figure 8, the treatment differences tended to remain rel-atively constant over the range of co-medication concen-trations, especially at the highest concentration range.

As hepatic AEs were to be expected based on the AEprofile of both bosentan and imatinib, and the combina-tion of both drugs is a potential source for additive oreven synergistic interaction in this regard, we investi-gated the potential impact of increased exposure of

Figure 9SGOT/SGPT vs. steady-state trough concentrations of imatinib. Only SGOT (A)/SGPT (B) values coinciding with a trough measurement at a steady stateare plotted (the number of SGOT/SGPT measurements exceeded the number of steady-state trough concentrations). The presence/absence of bosentanis indicated (shape and colour). The lines refer to loess smoothers through the respective groups. The shaded areas are 95% confidence intervals of thesmoothers. , bosentan; , no bosentan

PK analysis of imatinib, bosentan and sildenafil in patients with PAH

co-medications on the risk for hepatotoxicity. For this pur-pose, SGOT and SGPT concentrations were plotted againstthe corresponding trough concentrations of imatinib, bothin the presence or absence of bosentan (Figure 9). Overall,there was no clear tendency of increasing liver enzymeswith higher concentrations of imatinib. Similar conclusionshold when plotting liver enzymes vs. averaged bosentanconcentrations (results not shown).

Discussion

Population PK of imatinib in severe PAH were characterizedby similar CL/F and V/F compared with CML patients [16].They were also comparable with those of patients withGIST, although the estimated volume of distribution wassmaller in patients with GIST. Similar dosing regimensappear to be effective in these different disease areas.The major covariate relevant for imatinib in PAH was co-administration of bosentan, which decreased the exposureto imatinib and, therefore, does not pose an additionalsafety risk. This leads to lower steady-state metrics in thePAH population. Thus, typical trough values at steady-statein the CML/GIST populations are approximately 1000ngml–1. Our population model would predict a similar valuefor a typical patient not receiving bosentan. However,when taken across the study population, the averagedvalue is approximately 700ngml–1, which reflects bosentanusage in approximately 60% of the patients in our study.

In addition to co-administration with bosentan, a moreweakly significant relationship between haemoglobin and

apparent volume and clearance was identified. This isconsistent with findings in CML [16] and supported bythe known distribution of imatinib into erythrocytes[18, 19]. Although other covariates were not identified asstatistically significant in the covariate search analysis, itshould be stressed that our dataset was considerablysmaller than that investigated by Schmidli et al. [16] and,therefore, the results should not be misinterpreted as anabsence of effect (e.g. bodyweight on CL/F and V/F). Aneffect of WBC count on PK parameters was most likely notidentified because of the different populations (PAH vs.CML), their median WBC count (5.8 vs. 16.0 109 l–1) andvastly different distributions. We can further emphasizethat there was no evidence for a relevant PK difference be-tween different races, including Japanese and other Asianpatients. Obviously, this involved small numbers of patientsand such conclusions cannot be considered definitive.

When focusing on potential DDIs among imatinib,sildenafil and bosentan, we can first note that bosentanwas found to decrease sildenafil concentrations by ap-proximately 50% and sildenafil to increase bosentan con-centrations by approximately 50%, which is consistentwith previous reports [11, 20]. This is a first hintsupporting the validity of the sparse PK sampling ap-proach within a phase III study to elucidate such acomplex question of PK interactions among the threedrugs. In this analysis, it was also found that, on average,co-administration of imatinib resulted in increased con-centrations of bosentan by 50% and sildenafil by 66%.These results were confirmed by a retrospective compar-ison with the results of a dedicated DDI study requested

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by health authorities to quantify the PK impact of ima-tinib on co-administration of bosentan and sildenafil.This unpublished study estimated the increase in silden-afil exposure after administration of imatinib to be 70%(90% CI 43%, 103%), and the corresponding increase inbosentan exposure to be 40% (90% CI 23%, 59%). Thiswas again consistent with our own findings, which fur-ther testifies to the validity of the conclusions based ona sparse PK sampling approach to characterize PK inter-actions among the three drugs.

Interestingly, the effects of bosentan and imatinib onsildenafil cancelled out, bringing exposure close to thatobserved in the absence of both the drugs. Sildenafil didnot have an apparent effect on imatinib concentrations.However, bosentan, on average, decreased imatinib con-centrations by approximately 30% regardless of thepresence of sildenafil. The estimated combined effect ofsildenafil and bosentan was a decrease in imatinibconcentrations by 35%, indicating that the effect ofbosentan on the PK of imatinib is independent of sildenafilpresence, and sildenafil does not impact on the PK ofimatinib.

Following characterization of the different PK interac-tions among the three drugs, their clinical implicationswere assessed. The key finding that imatinib increased co-medication exposure immediately raises the questionwhether the efficacy of imatinib is caused by enhancingthe effect of bosentan and sildenafil via increasing their ex-posure. In this case, the effect of imatinib would be morepronounced at high concentrations of the respective co-medications. However, treatment differences between ima-tinib and placebo for change of 6MWD and change of PVRwere relatively constant across the entire concentrationrange for sildenafil and bosentan. Therefore, it is notexpected that an increase of either bosentan or sildenafilexposure towards higher concentrations would lead toimproved efficacy. Even in the absence of either sildenafilor bosentan, the effect of imatinib is as prominent as atthe highest concentrations of the respective drug, lendingstrong support to its intrinsic efficacy beyond merelyelevating the concentration of the co-medications due toDDIs. The ultimate proof would have been mono-administration to patients with PAH, but this would haverequired either withholding concomitant therapy frompatients already treated or substituting first line ther-apy, which has been proven effective, with a studydrug of unknown therapeutic value.

As bosentan concentrations were, on average,roughly doubled in the presence of sildenafil and ima-tinib, the next question to be answered was the clinicalrelevance of this increased exposure. With regard to po-tential increased risk of hepatotoxicity, elevation of trans-aminase levels has been reported, which occurred in 14%of patients receiving 250mg twice daily of bosentan, 4%of those receiving 125mg twice daily of bosentan and3% of those receiving placebo [13]. Overall, in our study,

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the occurrence of newly occurring or worsening liverfunction tests (defined as >3×ULN in SGOT, or >3×ULN in SGPT or >1.5 ×ULN in alkaline phosphatase or>1.5 ×ULN in total bilirubin) was 8% (eight of 102patients) in the imatinib group vs. 10% (10 of 98 patients)in the placebo group. When restricting to patients whowere taking bosentan, corresponding numbers were12% (seven of 59 patients) in the imatinib group and10% (five of 49 patients) in the placebo group. A trendof increased frequency of liver test abnormalities wastherefore seen in this subgroup, but such a difference ishardly conclusive based on the small patient numbers.In addition, an exposure-dependent increase to bosen-tan or imatinib was not evident for any of the liver trans-aminases in our study. In conclusion, there is no clearevidence of increased risk of elevated enzymes whenco-administering bosentan and imatinib. However, thelack of significant elevated enzymes in our relativelysmall study should not be taken as evidence for theoverall absence of this effect.

In conclusion, parameters of the population PKanalysis of imatinib in patients with severe PAH werefound comparable with those in patients with CML. Con-centrations of bosentan and sildenafil were elevated onco-administration with imatinib. Bosentan, on average,decreased imatinib concentrations, whereas sildenafilhad no effect on imatinib concentrations. Beyond merelyelevating the concentrations of the co-medications,further exploration of the data supports intrinsic efficacyof imatinib in this patient population. While our DDIassessment based on sparse PK data was quite robustand corroborated by a separate DDI study, furtherstudies would be needed to confirm the lack of clinicalrelevance of those PK interactions.

Competing Interests

This work was funded by Novartis Pharmaceuticals AG,Basel, Switzerland. All authors have completed theUnified Competing Interest form at http://www.icmje.org/coi_disclosure.pdf (available on request from thecorresponding author) and declare no support from anyother organization for the submitted work, no financialrelationships with any organizations that might have aninterest in the submitted work in the previous 3 yearsand no other relationships or activities that could appearto have influenced the submitted work. All the authorsare employees of Novartis and declare no conflict ofinterest. All the authors participated in the analysis andinterpretation of data and critical revision of themanuscript. The authors were assisted in the preparationof the manuscript by Snigdha Santra, SiddharthVishwakarma, Amit Bhat and Kevin Roche who providedmedical writing assistance and subsequent revisionsbased on authors’ feedback.

PK analysis of imatinib, bosentan and sildenafil in patients with PAH

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Supporting Information

Additional Supporting Information may be found in theonline version of this article at the publisher’s web-site:

Appendix S1Population PK modeling

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