Oral drug absorption and the Biopharmaceutics Classification System

Oral drug absorption and the Biopharmaceutics Classification System

J. DRUG DEL. SCI. TECH., 17 (4) 237-244 2007 Oral drug absorption and the Biopharmaceutics Classification System H. Lennernäs*, B. Abrahamsson, E.M. P...

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J. DRUG DEL. SCI. TECH., 17 (4) 237-244 2007

Oral drug absorption and the Biopharmaceutics Classification System H. Lennernäs*, B. Abrahamsson, E.M. Persson, L. Knutson Department of Pharmacy, Biopharmaceutics Research Group, Box 580, BMC, Uppsala University, SE-751 23 Uppsala, Sweden *Correspondence: [email protected] Bioavailability (BA) and bioequivalence (BE) play a central role in pharmaceutical product development and BE studies are presently being conducted for New Drug Application (NDAs) of new compounds, in supplementary NDAs for new medical indications and product line extensions, in Abbreviated New Drug Applications (ANDAs) of generic products and in applications for scale-up and post-approval changes. The Biopharmaceutics Classification System (BCS) has been developed to provide a scientific approach for classifying drug compounds based on solubility as related to dose and intestinal permeability in combination with the dissolution properties of the oral immediate release (IR) dosage form. The aim of BCS is to provide a regulatory tool for replacing certain BE studies by accurate in vitro dissolution tests. The aim of the present review is to present the status of BCS and discuss its future application in pharmaceutical product development. This will be discussed in relation to novel findings in human intestinal absorption, permeability and solubility. The future application of BCS is likely to be increasingly important if the BCS borders for certain Class II and III drugs are extended. The BCS is also a simple tool in early drug development to determine the rate-limiting step in the oral absorption process, which has facilitated the information between different experts involved in the overall drug development process. In the future, this increased awareness of a proper biopharmaceutical characterization of new drugs may result in drug molecules with a sufficiently high permeability, solubility and dissolution rate that will automatically increase the importance of BCS as a regulatory tool over time. Key words: Bioavailability – Bioequivalence – Biopharmaceutics Classification System.

discovery program. Consequently, in vitro and in silico methods were developed that were aimed at identifying shortcomings as early as possible in the drug discovery and preclinical phases. The benefit of this approach has led to a significantly reduced percentage of NCE (new chemical entity) failing projects related to pharmacokinetic problems [2]. However, in silico and in vitro predictions of intestinal absorption for modified release dosage forms still have to be significantly improved. This may be possible by further studies of the integrated in vivo situation along the gastrointestinal tract. Therefore, a discussion of the future value of these in vitro and animal data as efficient tools in drug discovery and early preclinical development has to be critical and sustained [3]. The Biopharmaceutics Classification System (BCS) is a scientific framework that categorizes drugs into four classes according to their solubility and intestinal permeability, the two fundamental parameters controlling intestinal drug absorption (Figure 1) [4, 5]. The aim of the current review is to summarize the oral drug absorption in humans and the application of the BCS. This will be focused on the use of human intestinal absorption studies.

The majority (~ 85%) of the 50 most-sold pharmaceutical products in the North American and European markets are given orally. This route of administration presently dominates drug therapy and is likely to continue to do so in the foreseeable future because in addition to causing minimal discomfort to the patient, it is safer, more efficient and more easily accessible than alternative routes such as intramuscular, subcutaneous, rectal and pulmonary delivery. However, despite these advantages many of the mechanisms of drug uptake following oral administration remain to be fully characterized. Identification of the rate-limiting step(s) in gastrointestinal absorption should contribute to overcome absorption barriers and to improve understanding of mechanisms for intra- and interindividual variability. This is important in the selection of suitable candidate molecules for drug development as well as in the design of new dosage forms that will satisfy future clinical needs [1]. Modification of oral pharmaceutical products is common in the life-cycle management in order to extend the exclusivity on already marketed drugs by developing new and innovative formulations. For instance, development of products with extended release profiles, including novel chronotherapeutic and intestinal targeting drug delivery approaches are in progress. It is also interesting that several new approaches are aiming to target the drug to a specific area and/or mechanisms present in the gastrointestinal tract. Such strategies include focusing on the bile acid transporters for treatment of hypercholesterolemia, management of inflammatory bowel disease (IBD) and new remedies for the different intestinal syndromes such as irritable bowel syndrome (IBS) [2]. Successful development of such non-absorbable drug products should provide a safer and more effective drug therapy for these diseases. Despite increasing investment in drug discovery and development in the pharmaceutical industry, productivity has deteriorated which is considered to be a consequence of several factors. Based on studies in the early 1990s it has been reported that half of the late-stage failures in drug development were attributed to poor pharmacokinetics and biopharmaceutics properties (39%) and animal toxicity (11%) [1]. As a consequence, all major pharmaceutical companies have included pharmacokinetic/toxicity (ADME-tox) screening criteria in their drug


Novel drugs need to have a sufficiently high bioavailability (F) leading to appropriate systemic exposure of the active substance in order to provide an efficient oral treatment of several diseases. It is important to recognize that bioavailability is defined as the fraction of a clinical dose that reaches the systemic circulation unchanged after dissolution, passage of a number of barriers, escaping metabolic enzymes in both gut and liver. It is important to recognize that bioavailability reflects the in vivo performance of oral dosage forms as well as the pharmaceutical product quality and safety. The latter has to be considered in the development of generic oral products, which should be interchangeable with the original product and provide the same clinical outcome. It also has to be contemplated when formulations and manufacturing processes are changed during the later clinical development phases or for marketed pharmaceutical products. In vivo investigations comparing the plasma exposure (AUC, Cmax and tmax) of two formulations of the same parent compound with 237

J. DRUG DEL. SCI. TECH., 17 (4) 237-244 2007

Oral drug absorption and the Biopharmaceutical Classification System H. Lennernäs, B. Abrahamsson, E.M. Persson, L. Knutson

indications and line-extensions, in ANDAs of generic products and in applications for scale-up and post-approval changes. For example, NDA bioequivalence studies may be required to compare different clinical formulations in pivotal clinical trials and products aimed for the market. The complexity and number of studies required is often boosted by the fact that several dose strengths might be included in the development process. In addition, bioequivalence documentation may also be needed to compare blinded and original comparator products in clinical trials. Thus, an NDA typically contains a multitude of bioequivalence studies and if that number can be decreased there should be both direct and indirect savings in the development cost [5].

Biopharmaceutical Classification System (BCS) 10

I 1



High Solubility Low Solubility High Permeability High Permeability



High Solubility Low Permeability

Low Solubility Low Permeability


0.01 1






Today it is considered that the future application of BCS will not be wide-spread since Class I substances are quite rare in pharmaceutical development. For instance, the proportion of Class I compounds in active development for oral immediate release (IR) formulations at AstraZeneca, a major research-based pharmaceutical company, was less than 10% year 2001 [5]. Thus, the usefulness of BCS for documentation of new chemical entities is limited from that perspective, but there is potentially greater use for BCS when regulating drug products on the World Health Organization (WHO) essential drug list. It contains 123 oral IR drug products and the percentage of the drugs classified as BCS Class I, Class II and Class III and Class IV were: 24.4% Class I, 17.1% Class II, 29.3% Class III and 12.2% Class IV. The remaining 17.1% of the drugs was not feasible to BCS classify [6]. It has been recognized that the present BCS application represents a deliberately conservative approach and proposals for extensions have been discussed since the original BCS publication. For example, in a recent paper it was suggested that the requirement of highest pH for the solubility measurements could be changed from 7.5 to 6.8 since the latter is more relevant for the pH in the stomach and upper small intestine [7]. This revision would thus somewhat relax the requirements for basic drugs. Another proposal in the paper by Yu was to reduce the high permeability definition from 90% to 85% fraction absorbed based on observations that many drugs that are considered completely absorbed provide experimental values below 90%, i.e. 90% seems to be too rigid a criteria considering the precision of the experimental methods [8]. Other more radical relaxations of criteria, such as including Class II drugs, using simulated intestinal media with bile acids or increasing volumes and further reducing the pH interval in solubility measurements, will most probably require further research and reanalysis of past experiences [9]. Waivers for Class III (high solubility, low permeability) drugs are scientifically justified and have been recommended assuming that dosage form excipients have no significant effect on the epithelial membrane permeability and the gastrointestinal transit [5, 10]. Extensions of BCS beyond the oral IR area have also been suggested, e.g. to apply BCS in the extended release area. However, this will provide a major challenge since the release from different formulations will interact in different ways with in vitro test conditions and the physiological milieu in the gastro-intestinal tract [5]. The situation for ER formulations would be further complicated by the need to predict potential food effects on the drug release in vivo [5]. Although the present use of BCS is limited, extensions of applications should clearly be made without jeopardizing the quality of products on the market. This would more likely be achieved within the area of oral IR than for ER formulations.

Solubility Figure 1 - The Biopharmaceutics Classification System (BCS) provides a scientific basis for predicting intestinal drug absorption and for identifying the rate-limiting step based on primary biopharmaceutical properties such as: solubility and effective intestinal permeability (Peff). The BCS divides drugs into four different classes based on their solubility and intestinal permeability. Drug regulation aspects related to in vivo performance of pharmaceutical dosage forms have been the driving force in the development of BCS. Guidance for industry based on BCS is mainly clarified when bioavailability/bioequivalence (BA/BE) studies can be replaced by in vitro bioequivalence testing (www.fda. gov/cder/guidance/3618fnl.htm).

the aim of verifying sufficient similarity, from a clinical perspective, between a new and a reference formulation are named bioequivalence (BE) studies. This more traditional approach for BE-studies is used for immediate, modified and extended release dosage forms. These generic drug products and their interchangeability are currently under intensive debate between politicians, pharmacists and physicians, since several governments have identified the development of generic drug products as one approach, among others, to decrease the escalating health care costs [5]. The rate and extent of drug absorption (fa) from a solid dosage form during its transit through the small and large intestine includes several steps: drug release and dissolution kinetics, stability and binding issues in the lumen, gastrointestinal transit time, and effective intestinal permeability (Peff) in different regions. The BCS provides a scientific basis for identifying the fundamental rate-limiting biopharmaceutical factors of intestinal drug absorption such as: solubility and effective intestinal permeability (Peff) (Figure 1). When combined with the in vitro dissolution kinetics of the crystals in the pharmaceutical product, the BCS takes into account three major absorption factors (solubility, dissolution and permeability) from which the rate and extent of intestinal drug absorption can be predicted. Furthermore, according to the BCS drugs are divided into four different classes based on their solubility and intestinal permeability. Drug regulation aspects related to in vivo performance of pharmaceutical dosage forms have been the driving force in the development of BCS. Guidance for industry based on the BCS mainly clarifies when bioavailability/bioequivalence (BA/BE) studies can be replaced by in vitro bioequivalence testing. This could reduce costs and time in the development process as well as reduce unnecessary drug exposure in healthy volunteers, which is normally the study population in BE studies. BCS allows a biowaiver for drugs which are highly soluble, completely dissolved and classified as having high intestinal permeability (Class I compounds). Accordingly, these IR pharmaceutical product are candidates for an in vitro assessment of BE as long as they are stable in the gastrointestinal tract and have a wide therapeutic window [4, 5]. There is a need to simplify the BE-testing by a scientific framework such as the BCS, since numerous BE studies are presently being conducted for NDAs of new compounds, in supplementary NDAs for new


Current research strategies in discovery and early pre-clinical pharmaceutical development recognize the importance of pharmaco238

Oral drug absorption and the Biopharmaceutical Classification System H. Lennernäs, B. Abrahamsson, E.M. Persson, L. Knutson

J. DRUG DEL. SCI. TECH., 17 (4) 237-244 2007

kinetic properties such as oral bioavailability, clearance, and a suitable elimination half-life. It has been clearly shown that physicochemical properties of drugs, such as molecular weight, hydrogen bonding potential and lipophilicity, are important factors that determine the successful passage of drug candidates through all stages of development [11]. It has also been concluded that limiting values of physicochemical properties are not historical artifacts but are under physiological control. Accordingly, increased understanding of the relation of various chemical descriptors and interplay between physiological and biochemical variables (such as system biology investigations of the in vivo situation) are important for increasing the success rate in drug development as well as for improving the understanding of factors explaining the significant intra- and interindividual variability seen in pharmacokinetics of several drugs [12-14]. The effective intestinal permeability (Peff) is a major determinant of fraction dose absorbed (fa) [4, 5]. It is well-established that some drugs may be transported by multiple mechanisms, such as passive diffusion and carrier-mediated processes in both absorptive and secretory directions [14, 15]. The expression and functional activity of the intestinal transport proteins and enzymes have been thoroughly investigated and our understanding of their contribution in oral drug delivery has significantly increased [14-17]. Undoubtedly, such knowledge will increase our understanding of the basis of inter-individual variability and regulation in drug response, both from a genomic and nongenomic perspective. The in vivo measured drug transport kinetics (Peff) represents the total transport (i.e., the macroscopic transport rate) of all parallel processes. Based on the last decade of success in oral drug delivery research it can be concluded that passive membrane permeability is the dominating intestinal drug absorption mechanism. Direct measurements of intestinal absorption, secretion and metabolism of drugs in humans are possible by regional intestinal perfusion techniques [18-22]. In general, three different clinical tools have been employed in the small intestine: (i) a triple-lumen tube including a mixing segment, (ii) a multilumen tube with a proximal occluding balloon (iii); and a multilumen tube (Loc-I-Gut) with two balloons occluding a 10-cm long intestinal segment [18]. The advantages and disadvantages of the various intestinal perfusion techniques are discussed elsewhere. In Figure 2 the Loc-I-Gut concept is displayed. This intestinal perfusion technique, Loc-I-Gut, has been widely applied to investigate drug absorption, presystemic metabolism, drug dissolution, in vitro-in vivo correlation, drug-drug interactions, herb-drug interactions, interindividual variability in GI physiology and disease mechanisms [18-22]. The Loc-I-Gut offers the possibility of directly investigating and predicting the integrated drug absorption processes in the human intestine. In addition, the Loc-I-Gut technique has been used to establish an in vivo human permeability database for a number of drugs that is a part of the proposed BCS for oral immediate release products (Table I) [18-23]. Figure 3 and Table II reveals the diversity in molecular structures and physicochemical properties, respectively, for the drug data set [12, 13]. The human in vivo Peff values, obtained during appropriate physiological conditions, provide the basis for establishing in vitro-in vivo correlation, which can be used when making predictions for gastrointestinal drug absorption as well as in setting bioequivalence standards [23, 24]. All together, this clearly indicates that human intestinal perfusion techniques are very useful for increasing our understanding of intestinal absorption, secretion and metabolism of drugs. Clinical studies of effective intestinal permeability (Peff), secretion and metabolism of various compounds, such as drugs, environmental pollutants and nutrients are rarely performed in vivo in humans even if experimental techniques are available (Figure 2). Direct measurements of compound transport and metabolism in the mesenteric and portal veins in humans are not possible for obvious reasons. Perfusion techniques do, however, offer considerable possibilities for measuring intestinal processes. Over the past 70 years different in vivo intestinal

Loc-I-Gut Stomach Drainage

Perfusate Entering Segment Perfusate Leaving Segment

Proximal Drainage


Figure 2 - Loc-I-Gut is a perfusion technique for the proximal region of the human jejunum. The multichannel tube is 175 cm long and is made of polyvinyl chloride with an external diameter of 5.3 mm. It contains six channels and is provided distally with two 40 mm long, elongated latex balloons, placed 10 cm apart each separately connected to one of the smaller channels. The two wider channels in the centre of the tube are for infusion and aspiration of perfusate. The two remaining peripheral smaller channels are used for the administration of marker substances and/or for drainage. At the distal end of the tube is a tungsten weight attached in order to facilitate the passage of the tube into the jejunum. The balloons are filled with air when the proximal balloon has passed the ligament of Treitz. Gastric suction is obtained by a separate tube. 14C-PEG 4000 is used as a volume marker to detect water flux across the intestinal barrier. Table I - BCS classification based on human permeability and dose number. Drug

Human permeability (x 104 cm/s)

Dose number*

BCS Class

α-methyldopa Amoxicillin Antipyrine Atenolol Carbamazepine Cephalexin Cimetidine Enalapril maleate Furosemide Hydrochlorothiazide L-Dopa Lisinopril Losartan potassium Metoprolol tartate Naproxen sodium Propanolol HCl Ranitidine HCl Terbutaline Valacyclovir HCl Verapamil

0.10 0.30 5.60 0.20 4.30 1.56 0.26 1.57 0.05 0.04 3.40 0.33 1.15 1.34 8.50 2.91 0.27 0.30 1.66 6.80

0.1 0.9 0.20 0.02 80 2 3 0.003 30 0.2 1.0 0.002 0.004 0.0004 0.06 0.01 0.01 0.01 0.02 0.004


*Dose number = (highest dose strength/gastric volume (250 ml))/solubility. **High permeability due to carrier mediated absorption, currently not included in BCS Class I.

perfusion techniques have been developed and the importance of such in vivo work has been clearly demonstrated [18-22]. The fundamental principle of an in vivo intestinal single-pass perfusion experiment is that the absorption (i.e., Peff) is calculated from the rate at which the compound disappears from the solution passing through the intestinal segment. An accurate determination of the Peff requires knowledge of the hydrodynamics, perfusion rate and the surface area of the per239

J. DRUG DEL. SCI. TECH., 17 (4) 237-244 2007

Oral drug absorption and the Biopharmaceutical Classification System H. Lennernäs, B. Abrahamsson, E.M. Persson, L. Knutson

Table II - The human effective permeability (Peff). Each Peff-value was determined in vivo in the proximal jejunum in humans with a single-pass approach at pH 6.5 (phosphate buffer) and during isotonic conditions at Uppsala University, Sweden. Compound

Log Peff

Pred log Peff




1. amoxicillin 2. antipyrine 3. atenolol 4. carbamazepine 5. creatinine 6. desipramine 7. enalaprilat 8. fluvastatin 9. furosemide 10. glucose 11. hydrochlorothiazide 12. ketoprofen 13. l-leucine 14. l-dopa 15. methyldopa 16. metoprolol 17. naproxen 18. piroxicam 19. propranolol 20. terbutaline 21. urea

-4.4 -3.35 -4.7 -3.38 -4.54 -3.35 -4.7 -3.62 -5.4 -3 -5.4 -3.07 -3.21 -3.47 -4.7 -3.92 -3 -3.21 -3.54 -4.52 -3.85

-5.63 -2.84 -4.63 -3.56 -3.84 -3.00 -4.50 -4.28 -5.03 -5.19 -5.12 -3.42 -4.11 -5.10 -5.06 -3.74 -3.35 -4.10 -3.53 -4.50 -4.51

365.40 188.23 266.34 236.27 113.12 266.39 348.40 411.47 330.74 180.16 297.73 254.29 131.18 197.19 211.22 267.37 230.26 331.35 259.35 225.29 60.06

154.38 26.5 88.05 41.59 67.31 16.16 102.07 81.37 124.3 113.63 132.6 54.23 66.31 105.51 102.56 57.81 48.18 90.9 39.22 76.38 77.5

5 0 4 2 2 1 3 3 4 5 4 1 3 5 5 2 1 2 2 4 4

Figure 3 - Diverse chemical structures for the drugs in which the human in vivo Peff have been determined.

for 20 compounds between human jejunum (in vivo perfusion) and in vitro Caco-2 monolayers have been compared (Figure 4). It is clearly shown that this in vitro model predicts permeability best for drugs with passive diffusion [23]. Drugs transported by carrier-mediated processes do not correlate in this comparison and is mainly due to poor expression of certain transport protein, such as peptide and amino acid carriers [23]. A direct in vivo assessment of the first-pass human gut wall metabolism in the gut wall was performed using the Loc-I-Gut technique and R/S-verapamil (log D6.5 2.7, octanol/phosphate buffer at pH 6.5;

fused intestinal segment. Fluid hydrodynamics are dependent on the perfusion technique applied, flow rate and GI motility [18-22]. One advantage of using Peff is that it can be applied regardless of the transport mechanism(s) across the intestinal mucosa. We have established a good correlation between in vivo determined Peff and historical data on fa for a large number of structurally diverse drugs (Table I and Figure 3). The human in vivo permeability can be applied to predict the fraction dose absorbed and assess in vitro-in vivo correlations which validate the use of different intestinal absorption models commonly applied in discovery and preclinical development [18-24]. Permeability data 240

Oral drug absorption and the Biopharmaceutical Classification System H. Lennernäs, B. Abrahamsson, E.M. Persson, L. Knutson

J. DRUG DEL. SCI. TECH., 17 (4) 237-244 2007

concentration gradient across the apical enterocyte membrane. A more plausible explanation may be that the concentration gradient in vivo across the intestinal barrier is provided by the extensive mesenteric blood flow. This would emphasize an important methodological aspect for intestinal perfusion techniques in which a determined Peff based on the disappearance rate from a perfused segment is not affected by extensive intracellular metabolism [25-28].

Human jejunum permeability ( x10-4 cm/s) at pH 6.5



10 Carrier-mediated : y = 1,1482x




R = 0,7854 15


13 12 18 10 11 9





Passive diffusion : y = 0,2857x 14


20 8


Dissolution of a drug molecule into the GI fluids is a prerequisite for drug absorption since the permeability of particulate material across the GI mucosa is negligible in the context of oral drug bioavailability. The dissolution process may significantly influence both the rate and extent of drug absorption following oral administration. It is well recognized today that the use of high-throughput techniques in the modern drug discovery process brings more lipophilic compounds into drug development, which make research in this traditional pharmaceutical scientific discipline even more crucial for successful pharmaceutical development [11, 29-38]. Drug dissolution, a kinetic parameter (amount/time), is the dynamic process by which crystals are dissolved in a fluid. Solubility is a thermodynamic parameter which describes an equilibrium state where the maximal amount of drug per volume unit is dissolved. The solubility of drugs is most often experimentally determined from the drug concentration in the liquid phase after adding excessive amounts of a solid drug substance to the test medium. This intrinsic solubility is affected by the solid-state properties of the drug, e.g. polymorphs, solvates, impurities and amorphous content. Equilibrium with the thermodynamically most stable solid state form, being the least soluble, should eventually be reached, but this might be a very slow process requiring several days. More short-term super-saturation phenomena may also occur, i.e. the measured solubility is much higher than the true saturation solubility during an initial phase before precipitation occurs from the supersaturated solution and equilibrium can be reached. Thus, although solubility is a simple concept, it is far from unproblematic to obtain robust data due to the indicated time dependence and effects of differences in solid-state properties as well as other sources of experimental variability. The drug dissolution rate could be determined by dispersing the powder in a test medium under suitable agitation or by studying the dissolution for a constant surface area by using the rotating-disc method [9, 31, 39]. The in vivo solubility and dissolution rate is affected by the unique physicochemical properties of the drug and the dosage form and by physiological factors such as pH, GI fluid composition and hydrodynamics. The in vivo situation is highly complex and the composition of intestinal fluids is likely to vary considerably due to meal ingestion, meal composition, gastric emptying, secretory output, intestinal transit/motility and other physiologic factors [32, 33, 39-43]. For instance, the effective surface area of the drug crystals will be affected by the wetting properties of the bile acids and other surface-active agents in the GI tract. The diffusivity of a drug molecule in the intestinal juice will be altered by changes in viscosity induced, for instance, by meal components. An increased dissolution rate could be obtained at more intense intestinal motility patterns or increased flow rates [41-43]. The effect on the dissolution of a low soluble compound of different hydrodynamic conditions, being relevant for the fasting and fed state, was recently investigated in an in vivo study [32,33, 44, 45]. It was found that the hydrodynamic conditions significantly affected both the rate and extent of bioavailability for slowly-dissolving unmilled drug particles whereas for more rapidly-dissolving micronized drug substances no effect was detected. The saturation solubility in the GI fluids could be affected by several factors such as pH, solubilization by bile acids or dissolution in lipid food components [32,33, 44]. The pH, which varies according to region as well as food intake, is a key factor for protelytic drugs with pKa values within or close to the




R = 0,8492

0,1 All Drugs : y = 0,4971x





R = 0,7276

0,01 0,01






Caco-2 permeability ( x10-4 cm/s) at pH 6.5

Figure 4 - A comparison of permeability data for 20 compounds between human jejunum (in vivo perfusion) and Caco-2 monolayers (in vitro). It is clearly shown that this in vitro model predicts permeability best for drugs with passive diffusion. When drugs (C) that are transported through carrier-mediated processes are compared there is a clear deviation, which is not surprising when human jejunum is compared with an in vitro model that has a colonic origin. (Data from reference [2, 7, 8, 14, 21, 22, 24])

pH 7.4; MW 455 Da) as the model compound for CYP 3A4 and Pgpmediated local intestinal kinetics [25]. The Peff for both enantiomers at each of the concentrations (4.0, 40, 120 and 400 mg/L) was 2.5 · 10-4, 4.7 · 10-4, 5.5 · 10-4 and 6.7 · 10-4 cm/s, respectively. A luminal concentration of 400 mg/L is expected to be achieved in the proximal small intestine after oral administration of a 100-mg dose of verapamil in an immediate release dosage form. The three other perfusate concentrations (4.0, 40, and 120 mg/L) applied representative fractions of the dose (30, 10 and 1%) left to be absorbed of a 100-mg dose [15, 25, 26]. The jejunal Peff of R/S-verapamil increased at higher luminal perfusate concentration, which is in agreement with a saturated Pgpmediated intestinal efflux. Furthermore, there was no difference in Peff between the R- and S-form of verapamil at any luminal concentration, which suggests that the efflux transport cannot discriminate between the R- and S-form of verapamil. However, the measured in vivo jejunal Peff was classified as high at all four perfusate concentrations (> 2.0 · 10-4 cm/s) [25, 26]. This study clearly shows that passive diffusion is the dominating transport mechanism for R/S-verapamil in the human intestine [25, 26]. Ketoconazole (at 40 mg/L) is a potent inhibitor of CYP 3A4 metabolism and a less potent Pgp modulator, which directly inhibited the CYP 3A4 metabolism but not the Peff of R/S-verapamil (120 mg/L) when they were coperfused in human jejunum [15, 25, 26]. It also demonstrates that intracellular metabolism has no effect on apical drug permeability. In this regard, it has been proposed that an intracellular CYP 3A4 metabolism may provide a more pronounced concentration gradient across the apical enterocyte membrane, which theoretically should increase the intestinal permeability. Since verapamil is transported mainly via passive diffusion and is subjected to extensive CYP 3A4 metabolism in the gut, it is considered to be a good model drug for investigating this issue in humans. However, an inhibition of small intestinal metabolism did not result in a decreased jejunal Peff. In addition, if intracellular CYP 3A4 metabolism had had a pronounced effect on jejunal Peff, S-verapamil would have had a significantly higher intestinal permeability, as its gut wall metabolism is more extensive than that of R-verapamil [26]. Altogether, this suggests that an extensive CYP 3A4 metabolism in the enterocyte in humans does not affect drug permeability across the apical membrane by increasing the 241

J. DRUG DEL. SCI. TECH., 17 (4) 237-244 2007

Oral drug absorption and the Biopharmaceutical Classification System H. Lennernäs, B. Abrahamsson, E.M. Persson, L. Knutson

physiological pH interval. The bile concentration in the intestine is always increased after food intake and mixed micelles with nutritional lipids are formed. However, significantly enhanced solubility due to solubilization can be achieved already at the concentrations prevailing under fasting conditions. The solubility of low soluble drugs has been shown to increase with increasing bile acid concentration in the media [32, 33, 45-50]. An example of the dramatic increase in solubility due to solubilization by bile components is given in Figure 5. The solubilization by bile acids increases by increased drug lipophilicity [32, 33]. An empirical algorithm for the solubilization ratio (SR) of drugs in bile acids has been developed (Equation 1), which indicates that for drugs with a log P < 2 no solubility improvement should be expected [5]. In a pharmacokinetic database of a total of 472 drugs, it was found that 235 drugs (50%) had a log P value higher than 2, which means that this process applies to a large part of the clinically used drugs. [51]. log SR = 2.09 + 0.64logP

Danazol fasted

Felodipine fasted

Danazol fed

Felodipine fed

450 400

Solubility (µg/ml)

350 300 250 y = 12.829x - 5.3085


R 2 = 0.9742 150 100 y = 1.1253x + 2.5572



R = 0.9212 0 -50










Bile acid concentration (mM)

Figure 5 - The solubility of two BCS Class II substances, felodipine and danazol as a function of bile salt concentration in fasted and fed intestinal fluids. An increase in the bile concentration enhanced the solubility of the two compounds. Felodipine shows a considerably steeper relationship to bile salt concentrations than danazole. The presence of dietary lipids in the fed intestinal fluids results in a much higher solubility of the two compounds than would be anticipated from the linear relationships.

Eq. 1

The amount of drug in solution that will affect the drug dissolution rate at “non-sink conditions” is dependent on the available volume controlled by oral intake, secretions and water flux over the GI wall. For instance, it has been approximated that the physiological volume of the small intestine varies from 50 to 1,100 mL, with an average of 500 mL in the fasted state [5, 41]. The drug concentration in the intestinal lumen is also dependent on the drug permeability, which will be of importance when the drug levels in the intestine approach the saturation level. It should, however, be noted that it is almost impossible to fully predict the in vivo dissolution rate due to the many factors involved, of which several have not yet been completely characterized. The introduction of new study techniques to directly follow drug dissolution in vivo in the human intestine should therefore be of importance [32, 33, 44]. For example, in vivo dissolution studies discriminated between the dissolution rates of the two different particle sizes of spironolactone, based on the intestinal perfusate samples. In addition, dissolution rates of carbamazepine obtained in vitro were significantly slower than the direct in vivo measurements obtained from the perfusion method. The higher in vivo dissolution rate was probably due to the efficient sink conditions provided by the high permeability of carbamazepine [32, 33, 44]. It is highly desirable in drug discovery and early drug development to predict the influence of the drug dissolution on oral absorption based on relatively simple measurements of dissolution or solubility [11-13, 51]. The primary variable for judgments of in vivo absorption is the dissolution rate rather than the solubility. Drug dissolution can be the rate-limiting step in the absorption process and thereby affect the rate of bioavailability, and often more importantly, it can also limit the extent of bioavailability when the dissolution rate is too slow to provide complete dissolution within the absorptive region(s) of the GI tract. However, most often solubility data are more readily available than dissolution rates for a drug candidate, especially in early phases when only minute amounts of drug are available, preventing accurate dissolution rate determinations. Consequently, predictions of in vivo effects on absorption caused by poor dissolution must often be made on the basis of solubility data rather than dissolution rate. Solubility in water of > 10 mg/mL in pH range 1-7 has been proposed as an acceptance limit to avoid absorption problems, while another suggestion is that drugs with water solubilities < 0.1 mg/mL often lead to dissolution limitations of absorption. It should be noted that these arbitrary limits may be conservative, i.e. the bioavailability of drugs with even lower solubility may not always be limited by drug dissolution. For example, a drug with much lower solubility, such as felodipine (0.001 mg/mL), provides complete absorption when administered in an appropriate solid dosage form [5]. This may be explained both by

successful application of dissolution-enhancing formulation principles and more favorable drug solubility in vivo owing to the presence of solubilizing agents such as bile acids. The BCS provides another model for biopharmaceutical interpretation of solubility data. If the administered dose is completely dissolved in the fluids in the stomach, which is assumed to be 250 mL (50 mL basal level in stomach plus administration of the solid dose with 200 mL of water), the drug is classified as a “high solubility drug” [4, 5, 10]. Such good solubility should be obtained within a range of pH 1-8 to cover all possible conditions in a patient and to exclude the risk of precipitation in the small intestine due to the generally higher pH there than in the stomach. Drug absorption is expected to be independent of drug dissolution for drugs that fulfill this requirement, since the total amount of the drug will be in solution before entering the major absorptive area in the small intestine, and the rate of absorption will then be determined by the gastric emptying of fluids. Such “highly soluble drugs” are advantageous in pharmaceutical development since no dissolution-enhancing principles are needed and process parameters that could affect drug particle form and size are generally not critical formulation factors. However, many low soluble drugs according to BCS have been developed into clinically useful products, i.e. this classification is hardly useful as a screen criterion in drug discovery. * It is clear that more mechanistic in vivo studies are needed in order to clarify the dynamic interplay between mechanisms of drug solubility, dissolution, permeability and metabolism in the human intestine during in vivo condition. There is also a need for further development of in vivo techniques for direct measurements of these processes from various regions along the GI tract in humans and to relate it to various physiological/pathophysiological conditions. This would certainly increase our knowledge of the important mechanisms as well as provide in vivo data for the development and validation of rapid and more reliable in vitro intestinal models and physiological based pharmacokinetic models.



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MANUSCRIPT Received 16 February 2007, accepted for publication 14 June 2007.