Imatinib mesylate (STI571, Glivec®, Gleevec® – Novartis, Basel, Switzerland) is a
potent competitive inhibitor of the tyrosine kinases associated with ABL (Buchdunger
et al, 1996; Druker et al, 1996), KIT (Buchdunger et al, 2000; Heinrich et al, 2000),
PDGFr (Buchdunger et al, 1996, 2000) and ARG (Okuda et al, 2001), which impedes the
interaction of ATP with the SH1 domain of these proteins (Schindler et al, 2000),
thereby inhibiting the phosphorylation of downstream target proteins. Imatinib is
a phenylaminopyrimidine derivative and represents the first of a new class of drugs
known as signal transduction inhibitors. Following initial phase I/II dose-escalation
studies with imatinib (Druker et al, 2001a, 2001b), subsequent studies have demonstrated
remarkable efficacy with minimal side effects mostly in Philadelphia-positive leukaemias
(Kantarjian et al, 2002; Ottmann et al, 2002; Sawyers et al, 2002; Talpaz et al, 2002;
O'Brien et al, 2003) but also in solid tumours (Joensuu et al, 2001). Currently, clinical
trials using imatinib with and without concomitant chemotherapy are being conducted
in a number of c-kit and PDGF-R-positive malignancies(Apperley et al, 2001; Fischer
et al, 2001; Johnson et al, 2002).
Imatinib is a competitive inhibitor of CYP3A4, CYP2D6 cytochrome P450 isoenzymes as
well as CYP2C9, CYP3A5 and CYP4A to a lesser extent (Novartis, unpublished data).
Coadministration of inhibitors of CYP3A4 with drugs known to be substrates of this
enzyme could potentially affect the pharmacokinetic parameters of the substrate drug,
and could be also responsible for considerably increasing its side effects (Desager
and Horsmans, 1996). Simvastatin, an inhibitor of 3-hydroxy-3-methylglutaryl-coenzyme
A (HMG-CoA) reductase, is used as a lipid-lowering agent. It is uniquely metabolised
by CYP3A4, is well tolerated and is therefore commonly recommended as the model drug
for testing drug interactions involving CYP3A4 substrates (US Department of Health
and Human Services, 1999). Drug interactions between CYP3A4 substrates such as simvastatin
and CYP3A4 inhibitors are potentially clinically important and have been reported
to potentially enhance the risk of myopathy and rhabdomyolysis (Walker, 1989; Todd
and Goa, 1990; Berland et al, 1991; Smith et al, 1991; Garnett, 1995; Meier et al,
1995; Goodman & Gilman's, The Pharmacological Basis of Therapeutics, 2000). Accordingly,
the prescription instructions for HMG-CoA reductase inhibitors often suggest caution
regarding the potential of occurrence of drug interactions with substrates, inhibitors
and inducers of CYP3A compounds (Walker, 1989).
We therefore hypothesised that coadministration of imatinib, as an inhibitor of the
microsomal CYP3A4 enzyme system, could affect the elimination rate of simvastatin.
The present study was undertaken to assess this potential pharmacokinetic interaction
by evaluation of the simvastatin plasma concentration vs time profiles after coadministration
with imatinib (US Department of Health and Human Services, 1999) and its effects on
safety and tolerability in patients with chronic myeloid leukaemia (CML).
MATERIALS AND METHODS
Patient population
In an open-label, nonrandomised, one-sequence study, 20 adult patients with CML who
were haematologically or cytogenetically resistant or refractory to interferon-α,
or intolerant of interferon-α were enrolled. There were 10 male and 10 female patients
the mean age (±s.d.) was 50.5 years (±13.4 years), weight ranged from 53 to 111 kg,
and height from 158 to 192 cm. None of the patients had any past or present medical
conditions that could affect the study results. Each patient gave written informed
consent before taking part of the study, which was approved by the ethics committee
of the Landesärztekammer Rheinland-Pfalz, Mainz (Germany) and of the University of
Newcastle/Royal Victoria Infirmary (UK). The study was conducted in agreement with
the declaration of Helsinki, as amended in Tokyo, Venice, Hong-Kong and Somerset West.
Drug interaction studies performed in healthy volunteers commonly use a crossover
design. However, one of the requirements of the present study was to test interactions
with simvastatin at steady-state levels of imatinib. As it usually requires approximately
7 days to reach serum steady-state levels of imatinib, it was considered unethical
to perform the study in healthy volunteers and therefore the study was conducted in
patients with CML. As a corollary of this restriction, practical and ethical issues
(e.g., wash-out phase) prevented the study being performed as a crossover design.
The use of concomitant medications that could potentially alter the integrity of the
PK analysis (e.g., altered absorption, distribution) was forbidden. The patients were
asked to refrain from strenuous physical exercise (e.g., weight training, aerobics,
football) for 7 days before dosing until after the study completion evaluation, from
alcohol for 72 h before dosing until after the study completion evaluation and from
intake of xanthine (e.g., caffeine) or grapefruit (known as a CYP3A4 inhibitor (Schmiedlin-Ren
et al, 1997; Lilja et al, 1998; Kane and Lipsky, 2000))-containing food or beverages
48 h before dosing and during the whole study.
Study design
On study days 1 and 8, patients reported to the study site around 1 h prior to dosing
for baseline evaluations and were kept at the centre until 12 h postdosing (Table
1
Table 1
Blood sampling schedule for pharmacokinetic analyses
Pretreatment period
Pretreatment period
Treatment period
Treatment period
Treatment period
Post-treatment
Days −21 to −2: screening evaluations
Period I:
Period I:
Period II:
Period III:
End of study evaluations 24 h after dosing day 8
Day −1: baseline evaluations
Day 1: 40 mg simvastatin
Days 2–7: 400 mg Glivec od
Day 8: 400 mg Glivec od+40 mg simvastatin
PK sampling
PK sampling
). At 24 h after dosing, the patients reported again to the study site for the 24 h
blood sampling (study days 2 and 8) and study completion evaluations (study day 9).
Blood samples for determination of simvastatin plasma concentrations were taken up
to 24 h after dosing on study days 1 and 8. Imatinib was administered daily starting
at day 2 at a dose of 400 mg (supplied as 100 mg hard gelatine capsules). Simvastatin
(40 mg tablets of Denan® – Boehringer Ingelheim) for both centres was purchased by
the pharmacist of the University of Mainz's Hospital. On study days 1 and 8, 40 mg
of oral simvastatin was administered immediately after a low fat breakfast (on day
8, simvastatin and imatinib were given at the same time). No fluid intake apart from
the fluid given at the time of drug intake was allowed until 2 h after dosing. Owing
to the inherent risk of either reduced activity or enhanced toxicity of the concomitant
medication and/or imatinib, drugs known to be metabolised by the same CYP450 isoenzymes
as imatinib, were forbidden. Allopurinol 300 mg daily (an inhibitor of CYP2C9/10 (Yokochi
et al, 1982; Veronese et al, 1991; Kane and Lipsky, 2000)) was recommended for patients
with WBC 20.0 × 109 l−1. Following the PK study, patients continued with therapeutic
imatinib at the standard dose.
Blood sampling
All blood samples were taken by either direct venepuncture or an indwelling cannula
inserted in a forearm vein at predose (0 h), 0.5, 1, 2, 3, 4, 6, 10, 12 and 24 h after
dosing on days 1 and 8 (Table 1). Immediately after the blood was drawn, each tube
was inverted gently several times to ensure the mixing of tube contents (e.g., anticoagulant)
and prolonged sample contact with the rubber stopper was avoided. The upright tube
was kept on ice, and within 30 min the sample was centrifuged at 3 and 5°C for 10 min
at approximately 1500 g. Immediately after centrifugation, at least 2 ml plasma was
transferred to a polypropylene screw-cap tube put on dry ice. The tubes were kept
frozen at ⩽−18°C pending analysis.
Drug analysis
Simvastatin and simvastatin hydroxy acid with lovastatin as internal standard were
determined in plasma by LC/MS/MS. The LC/MS/MS analyses were carried out on a Sciex
API3000 mass spectrometer. The instrument was operated in the ESI mode (positive ion
for drug, negative ion for metabolite) with selected reaction monitoring. LC was performed
on a Shimadzu LC system operated in isocratic mode with a 2.0 × 50 mm2 C-18 column.
Samples were prepared using a solid-phase extraction procedure. All concentrations
are reported in terms of the free acid form of simvastatin and simvastatin hydroxy
acid.
Data analysis
All completed patients were included in the pharmacokinetic data analysis. For plasma
concentrations of simvastatin the following parameters were determined: AUC(0–t) (area
under the concentration–time curve from time zero to t), AUC(0–∞) (area under the
concentration–time curve from time zero to infinity), C
max (maximum plasma drug concentration), t
max (time to reach maximum concentration following drug administration), t
1/2 (elimination half-life associated with terminal slope of a semilogarithmic concentration–time
curve), V
z
/f (apparent volume of distribution based on terminal phase of plasma concentration–time
curves) and CL/F (total body clearance of drug from the plasma), in order to assess
the effects of imatinib on the PK of simvastatin.
Statistical analysis
The following pharmacokinetic parameters were used to assess an interaction of imatinib
on simvastatin: AUCinf, AUCall, C
max, V
z
/f, CL/f, t
1/2 and t
max. With the exception of t
1/2 and t
max, parameters were ln-transformed prior to analysis. Treatment differences were
assessed by t-tests. The means of differences of ln-transformed data together with
90% confidence intervals were then antilogged in order to get confidence intervals
for the ratio ‘simvastatin+imatinib/simvastatin’. An interaction of imatinib with
simvastatin was assumed, if these confidence intervals were not included in the ‘no-effect’
interval (0.80, 1.25). T
max was analysed nonparametrically. The alpha-level was set to 0.05 and no alpha-adjustment
was made for multiple testing.
RESULTS
Drug safety and tolerability
In total, 14 (70%) of the 20 recruited patients reported a total of 30 adverse events.
All but one adverse events were rated by the investigators as mild (grade 1) to moderate
(grade 2). Of these, 12 patients had at least one adverse event grade 1, and four
patients at least grade 2. Only one patient experienced a grade 3 left leg cellulitis,
but this was assessed as not related to the study drugs. No deaths occurred during
the course of the study and none of the adverse events resulted in discontinuation
from the study. The most common adverse events reported were neurological symptoms
(headache, insomnia), gastrointestinal symptoms (nausea, loose stool), and musculoskeletal
symptoms (myalgia, muscle cramps cramping, pain in limb).
Pharmacokinetics of simvastatin
The main pharmacokinetic parameters of simvastatin and its hydroxy acid metabolite,
for the 20 CML patients determined by noncompartmental model analyses, are listed
in Tables 2
Table 2
Simvastatin PK parameters following oral administration of 40 mg simvastatin alone
and combined with oral administration of 400 mg imatinib
Simvastatin plus imatinib
Simvastatin alone
t
max (h)a
1.0 (0.5–3.0)
1.0 (0.5–4.0)
C
max (ng ml−1)
42.3±25.8
23.3±23.8
t
1/2
2.7±1.3
1.4±0.8
AUC(0–all) (ng h ml−1)
136.4±113.6
45.5±61.1
AUC(0–∞) (ng h ml−1)
137.7±110.2
47.2±60.4
V
z
/F(l)
1543.0±810.9
3115.9±2749.9
CL/F (l h−1)
504.1±431.8
2000.3±1975.3
a
Median (range). All unflagged values are mean±s.d. Mean and s.d. values are shown.
and 3
Table 3
PK parameters of simvastatin hydroxy acid following oral administration of 40 mg simvastatin
alone and combined with oral administration of 400 mg imatinib
Simvastatin plus imatinib
Simvastatin alone
t
max (h)a
1.0 (0.5–3.0)
1.0 (0.5–4.0)
C
max (ng ml−1)
24.9±19.3
14.5±13.3
t
1/2
3.3±1.4
2.4±1.1
AUC(0–all) (ng h ml−1)
116.1±104.2
44.3±41.7
AUC(0–∞) (ng h ml−1)
119.9±106.0
51.9±39.9
Mean and s.d. values are shown. See Table 2.
. The mean and standard deviation for each parameter are given for the two treatment
periods in which simvastatin was administered. Figures 1
Figure 1
Plasma concentrations (mean+s.d.) of simvastatin following oral administration of
simvastatin alone (○) and combined with imatinib (□).
and 2
Figure 2
Plasma concentrations (mean+s.d.) of simvastatin hydroxy acid following oral administration
of simvastatin alone (•) and combined with imatinib (▪).
show the mean plasma concentrations of simvastatin and its metabolite (simvastatin
hydroxy acid), respectively, following either oral administration of simvastatin alone
or combined with oral administration of imatinib. Figure 3
Figure 3
Comparison of AUC(0–∞) of simvastatin following oral administration of simvastatin
alone and combined with Glivec® (STI571). Simvastatin alone shown in open columns,
simvastatin plus imatinib shown in black columns.
shows the comparison of plasma AUC(0–∞) of simvastatin following oral administration
of simvastatin alone and combined with oral administration of imatinib in 20 subjects.
Following imatinib coadministration, the mean simvastatin C
max, AUC(0–all) and AUC(0–∞) increased significantly by two-to-three-fold (P<0.001).
There was a statistically significant decrease in CL/F with a mean reduction of 70%
(P<0.001). With regard to metabolites, the mean C
max and AUCs of simvastatin hydroxy acid also increased significantly by two-to-three-fold
(P<0.001) after imatinib treatment (Table 3 and Figure 2). The coefficient of variation
(CV) for C
max and AUCs showed considerable interpatient variation. The mechanism for this variability
is not clear yet but could be attributed to interpatient variations in CYP3A4 activity.
Compliance to imatinib treatment and plasma concentration at steady state were checked
by the analysis of the plasma samples taken on study days 8 and 9 in the morning prior
to administration. The mean plasma imatinib trough concentrations were similar on
day 8 (1268 ng ml−1) and day 9 (1182 ng ml−1) indicating that PK steady state for
imatinib was reached in those patients after 6-day oral doses.
DISCUSSION
This study was performed to determine whether imatinib could alter the pharmacokinetics
of a single dose of simvastatin when given concomitantly in patients with chronic
myeloid leukaemia. This has important implications because of potential interactions
of imatinib with commonly prescribed drugs in the clinic.
The major route of degradation of simvastatin within the body is by cytochrome P450
3A4-mediated biotransformation (Vickers et al, 1990; Prueksaritanont et al, 1997)
although the drug can be converted reversibly to simvastatin hydroxy acid by esterases.
From in vitro drug interaction studies, CYP3A4 was also found to be the major human
P450 enzyme involved in the microsomal biotransformation of imatinib (data not shown).
Simvastatin inhibits HMG-CoA reductase causing decreases in intrahepatic cholesterol
and upregulation of LDL-receptors with enhanced clearance of LDL and other apolipoprotein
B containing lipoproteins from the circulation. It appears that these interactions
do not have a relevant clinical effect on the efficacy of the HMG-CoA reductase inhibitors
to reduce the cLDL from the plasma, but the concomitant administration of HMG-CoA
reductase inhibitors and cyclosporine, fibrate or nicotinic acid may enhance the risk
of myopathy or rhabdomyolysis (Berland et al, 1991; Smith et al, 1991; Meier et al,
1995).
The coadministration of simvastatin with imatinib (400 mg) was well-tolerated and
no major safety concerns were reported in this study. No clinically significant abnormalities
in laboratory values, vital signs or ECGs were reported. The majority of the adverse
events were assessed as grade 1/2 and no myopathy or rhabdomyolysis occurred. Only
one grade 3 left leg cellulitis (which required hospitalisation) was reported but
was not related to the study drugs. This study shows that coadministration of imatinib
increased the mean C
max value of simvastatin two-fold and the AUC(0–∞) value three-fold compared with
simvastatin alone and the mean half-life of simvastatin was prolonged from 1.4 to
2.7 h when given together with imatinib. This indicates an inhibition of CYP3A4 by
which the oxidative biotransformation of simvastatin to other metabolites is primarily
mediated. It was also observed that the formation of simvastatin hydroxy acid from
simvastatin by esterases is not prevented by imatinib, which explains the increases
in both simvastatin and simvastatin hydroxy acid concentrations. This would suggest
that in the presence of imatinib, plasma levels of standard doses of drugs which are
degraded by the CYP3A4 system (Table 4
Table 4
Substrates of cytochrome P450 enzymes CYP3A3/4
Substrates: imatinib may increase the potency of these drugs
Acetaminophen, Alfentanil, Alosetron, Alprazolam, Amiodarone, Amitriptyline (minor),
Amlodipine, Anastrozole, Androsterone, Antipyrine, Astemizole, Atorvastatin, Benzphetamine,
Bepridil, Bexarotene, Bromazepam, Bromocriptine, Budesonide, Bupropion (minor), Buspirone,
Busulphan, Caffeine, Cannabinoids, Carbamazepine, Cerivastatin, Cevimeline, Chlorpromazine,
Cimetidine, Cisapride, Citalopram, Clarithromycin, Clindamycin, Clomipramine, Clonazepam,
Clozapine, Cocaine, Codeine (demethylation), Cortisol, Cortisone, Cyclobenzaprine
(demethylation), Cyclophosphamide, Cyclosporine, Dapsone, Dehydroepiandrostendione,
Delavirdine, Desmethyldiazepam, Dexamethasone, Dextromethorphan (minor, N-demethylation),
Diazepam (minor;hydroxylation, N-demethylation), Digitoxin, Diltiazem, Disopyramide,
Docetaxel, Dofetilide (minor), Dolasetron, Donepezil, Doxorubicin, Doxycycline, Dronabinol,
Enalapril, Erythromycin, Estradiol, Ethinyl Estradiol, Ethosuximide, Etoposide, Exemestene,
Felodipine, Fentanyl, Fexotenadine, Finasteride, Fluoxetine, Flutamide, Glyburide,
Granisetron, Halofantrine, Hydrocortisone, Hydroxyarginine, Ifosfamide, Imipramine,
Indinavir, Isradipine, Itraconazole, Ketoconazole, Lansoprazole (minor), Letrozole,
Levobupivicaine, Lidocaine, Loratadine, Losartan, Lovastatin, Methadone, Mibefradil,
Miconazole, Midazolam, Mifepristone, Mirtazapine (N-demethylation), Montelukast, Navelbine,
Nefazodone, Nelfinavir, Nevirapine, Nicardipine, Nifedipine, Niludipine, Nimodipine,
Nisoldipine, Nitrendipine, Omeprazole (sulfonation), Ondansetron, Oral contraceptives,
Orphenadrine, Paclitaxel, Pantoprazole, Pimozide, Pioglitazone, Pravastatin, Prednisone,
Progesterone, Proguanil, Propafenone, Quercetin, Quetiapine, Quinidine, Quinine, Repaglinide,
Retinoic acid, Rifampin, Risperidone, Ritonavir, Salmeterol, Saquinavir, Sertindole,
Sertraline, Sibutramine, Sildenalfil citrate, Simvastatin, Sirolimus, Sufentanil,
Tacrolimus, Tamoxifen, Temazepam, Teniposide, Terfenadine, Testosterone, Tetrahydrocannabinol,
Theophylline, Tiagabine, Tolterodine, Toremifene, Trazodone, Tretinoin, Triazolam,
Troglitazone, Troleandomycin, Venlafaxine (N-demethylation), Verapamil, Vinblastine,
Vincristine, Warfarin (R-warfarin), Yohimbine, Zaleplon (minor pathway), Zatoestron,
Zileuton, Ziprasidone, Zolpidem, Zonisamide.
Simvastatin in bold is a substrate for CYP3A4 and in this study, coadministration
with imatinib resulted in significantly increased mean maximum concentration and area
under concentration–time curve values for simvastatin. One might expect coadministration
with imatinib to increase the potency of these drugs. Caution is also required with
drugs which induce or inhibit CPY3A4. Adapted from Cytochrome P-450 Enzymes and Drug
metabolism. (In: Lacy CF, Armstrong LL, Goldman MP, Lance LL (eds) Drug Information
Handbook, 8th edn. Hudson, OH: LexiComp Inc. 2000: pp 1364–1371).
and see also:
http://medicine.iupui.edu/flockhart/) may be increased. For example, one might predict
that the effects of warfarin, digoxin, certain antihypertensive agents (e.g., diltiazem,
nifedipine, verapamil), steroids, benzodiazepines and other drugs commonly used in
the practice of haematology (e.g., busulphan, cyclosporine, cyclophosphamide, doxorubicin
etoposide, vincristine) could be enhanced and appropriate vigilance to avoid undesirable
effects should be exercised. In addition, concomitant use of simvastatin or other
HMG-CoA reductase inhibitors with imatinib may increase the risk of myopathy or rhabdomyolysis
and again caution is required.
The design of the study allows only limited interpretation of the effects of simvastatin
on plasma levels, and perhaps therefore efficacy, of imatinib. However there were
no apparent effects of simvastatin on the PK of imatinib in these 20 patients, although
more detailed PK studies would be required to resolve this issue definitively. Although
this study was not designed to assess the potential relationships between the efficacy
of imatinib and pharmacokinetics parameters, this important question is being addressed
by ongoing population PK/PD modelling analyses within the context of ongoing phase
II and phase III studies.
In conclusion, the coadministration of imatinib (400 mg) at steady state with 40 mg
simvastatin significantly (P<0.001) increases the exposure (C
max and AUCs) to simvastatin by two-to-three-fold. This effect is most likely the
result of the inhibition of CYP3A4-mediated metabolism of simvastatin in the liver
and has implications for the monitoring of concomitant therapies in patients being
treated with imatinib. Caution is therefore required when administering imatinib with
CYP3A4 substrates with a narrow therapeutic window.