Pharmacokinetics of gefitinib in humans: The influence of gastrointestinal factors

Abstract

Purpose

To investigate whether differences in plasma pharmacokinetic profiles of gefitinib between healthy subjects having normal (N; t_1/2 > 20 h) and altered (A; t_1/2 < 20 h) pharmacokinetic (PK) profiles might be explained by inter-individual variability in gastric emptying and/or precipitation/dissolution of gefitinib in the proximal small intestine.

Methods

One hundred healthy male subjects were screened to enable identification of subjects with the two PK profiles. Twenty five subjects from the screening were subsequently enrolled in an intubation study where a 250 mg gefitinib dispersion preparation (IRESSA^®, AstraZeneca) was administered directly into the stomach. Intestinal fluid samples were withdrawn continuously for 180 min post-dose using the Loc-I-Gut catheter positioned in the jejunum. The crystalline form of gefitinib was determined using Raman microscopy.

Results

There were no differences between normal and altered subjects with regard to gastric emptying or the precipitation/dissolution of gefitinib in jejunal fluid. Due to difficulties in crystalline identification in the jejunal fluid samples, only the same crystalline form as the dosed form was identified.

Conclusions

There was no pronounced difference in gastric emptying, precipitation and re-dissolution of gefitinib in proximal human jejunum between normal and altered subjects. Other mechanism(s) are also likely to be important in explaining the inter-individual differences in plasma exposure to gefitinib, such as polymorphism in various metabolic enzymes and/or transport proteins. However, the difference between altered and normal subjects cannot be easily explained and it is likely a multifactorial explanation including low jejunal pH, increased expression of enzymatic and transporter activity and rapid small intestine transit.

Introduction

The epidermal growth factor receptor (EGFR) is over-expressed or dysregulated in a variety of solid tumours and plays a crucial role in their development, through involvement in increased cell proliferation and inhibition of apoptosis as well as enhancement of tumour vascularisation. Drug targeting of intracellular EGFR tyrosine kinase activity by gefitinib has resulted in a reduction in tumour growth and tumour cell death (Pao et al., 2004). Gefitinib (IRESSA^®, AstraZeneca) is an anilinoquinazoline (4-quinazolinamine, N-(3-chloro-4-fluorophenylamino)-7-methoxy-6-[3-(4-morpholinyl) propoxy]) with a molecular weight of 446.9 Da. It is a lipophilic di-basic compound with a log D_{pH 7.4} of 3.9 and possesses two pK_a-values of 5.28 and 7.17, and accordingly exhibits a pH-dependent solubility in gastrointestinal fluid. The solubility of gefitinib in aspirated human gastric fluid (pH 5.0) and intestinal fluid (pH 7.0) was 4.98 and 0.085 mg/ml, respectively (data on file, AstraZeneca). The oral bioavailability following an single dose of 250 mg gefitinib (IRESSA^® tablet) as a free base in healthy male volunteers was 57% and manipulation of gastric pH to elevate it above five resulted in a 47% reduction of the relative bioavailability (data on file, AstraZeneca) (Swaisland et al., 2005). The maximum plasma concentration after a 250 mg dose gefitinib is typically observed after 5 h (Swaisland et al., 2005). As the in vivo intestinal permeability of gefitinib is predicted to be high based on data obtained in the Caco-2 model, we consider that the solubility and dissolution rate of gefitinib along the intestine might be the rate-limiting step in the absorption process. This is further supported by the observation that gefitinib undergoes rapid dissolution in acidic media but the solubility drops as pH increases to neutral intestinal pH (data on file, AstraZeneca). In summary, gefitinib is a Biopharmaceutical Classification System (BCS) class II compound, as the intestinal permeability is high and the solubility (and dissolution rate) is low along the intestine (Amidon et al., 1995).

Gefitinib is extensively distributed in the body and is mainly eliminated via metabolism and less than 0.5% (doses ranging from 1 to 75 mg) is excreted as unchanged gefitinib in urine (Swaisland et al., 2001). After a single oral dose of 50 mg radio-labelled gefitinib, 3.4 ± 1.0% was recovered in urine and 86.3 ± 6.6% (mean ± S.D.) was recovered in faeces of which only 12.1% was as parent drug in humans after sampling up to 240 h post-dose (Mckillop et al., 2004). Gefitinib and its metabolites are most likely eliminated by biliary and/or direct intestinal secretion.

The disposition kinetics of a 250 mg dose of gefitinib is characterized by a plasma half-life of about 39.7 h (range 26.9–83.2 h) in healthy volunteers (Swaisland et al., 2005). An analysis of the pharmacokinetic data from all healthy volunteer data generated during the clinical development of gefitinib showed that the majority of individuals orally dosed with gefitinib had post-absorptive plasma concentration time profiles that were biphasic with a terminal half-life in the order of 30 h. However, approximately 20% of the volunteer population had post-absorptive concentration–time profiles that were more monophasic in nature with an apparently shorter terminal half-life of about 10 h (data on file, AstraZeneca). Plausible explanations for the difference between these two groups with regards to plasma exposure might be differences in gastric emptying, drug dissolution and/or precipitation that affect existing absorption rate limitations and/or differences in metabolic capacity (i.e. clearance).

The Loc-I-Gut technique is a well established in vivo method for gastrointestinal intubation in humans and used for various biopharmaceutical and pharmacokinetic applications (Knutson et al., 1989, Petri et al., 2003, Lindahl et al., 1996, Tannergren et al., 2003, Sandstrom et al., 1998, Bonlokke et al., 1997, Bonlokke et al., 2001, Bonlokke et al., 1999, Lennernas et al., 1992). In the present study the Loc-I-Gut tube was introduced orally and once positioned in the jejunum a semi-open segment was created through inflation of the distal balloon, which offers well-defined conditions for in vivo investigations of dissolution and absorption of drugs (Fig. 1) (Bonlokke et al., 1997, Lennernas et al., 1992, Lindahl et al., 1997, Knutson et al., 1989). The present study was the first direct in vivo investigation in humans of the effect of precipitation and was aimed at studying changes of the drugs crystal structure on gastrointestinal absorption variables and the plasma exposure for a BCS class II drug such as gefitinib.

The main purpose of the study was to investigate whether the difference in plasma pharmacokinetic profiles between normal and altered healthy subjects might be explained by inter-individual variability in gastric emptying and/or precipitation/dissolution of gefitinib in the proximal small intestine by using a modification of an established in vivo intubation technique. The study also aimed to investigate the crystal form of the precipitated gefitinib in human jejunum by using Raman microscopy.