Biopharmaceutical Classification System in In-vitro / In-vivo Correlation: Concept and Development Strategies in Drug Delivery

Drug development is a very laborious and expensive process. One of the major reasons for failure during the clinical phases of drug development is inadequate pharmacokinetic data on the drug candidate. Therefore, it would be advantageous if the pharmacokinetic properties of drug candidates be predicted beforehand. One major obstacle in making such predictions is the inability to appropriately scale the in-vitro data to the in-vivo situation. Results from in-vitro in-vivo correlation (IVIVC) studies have been used to select the appropriate excipients and optimize the manufacturing processes for quality control purposes, and for characterizing the release patterns of newly formulated immediate release, and modified-release products relative to the references. In recent years, the concept and application of the IVIVC for pharmaceutical dosage forms have been a major focus of attention in the pharmaceutical industry, academia and regulatory agencies. Hence, this article highlights the importance of appropriate selection of IVIVC level with respect to the Biopharmaceutical Classification System (BCS) and also covers examples of BCS-based IVIVCs of drugs/products with different types of release profiles.


INTRODUCTION
In recent years, the concept and application of the in vitro/in vivo correlation (IVIVC) for pharmaceutical dosage forms have been a main focus of the pharmaceutical industry, academia and regulatory bodies [1].
A regulatory guidance for both immediate (IR) and modified release (MR) dosage forms has, therefore, been developed by United States Food and Drug Administration (FDA) to minimize the need for bioavailability studies as part of formulation design and optimization [2]. In vitro dissolution testing of solid oral dosage forms serves as a very important tool in drug development for selecting and optimizing formulations, studying drug-release mechanisms, ensuring batch-to-batch consistency, monitoring stability and demonstrating bioequivalence. Additionally, for the development of modified release (MR) dosage form. the focus of dissolution testing in the early stage of drug development is to provide predictive estimates of drug release in respect to in vivo performance of a drug product [3]. Thus no costly in vivo bioequivalence testing is required [4].
This review essentially addresses FDA guidelines for development, evaluation, and application of in vitro/in vivo correlations and highlights the importance of appropriate selection of IVIVC level with respect to Biopharmaceutical Classification System (BCS). The review also looks at examples of BCS based IVIVCs of drugs/products with different type of release profiles.

IN VITRO/IN VIVO CORRELATIONS
IVIVC has been defined by the FDA as "a predictive mathematical model describing the relationship between an in-vitro property of a dosage form and an in-vivo response" [4]. Generally; the in-vitro property is the rate or extent of drug dissolution or release while the in-vivo response is the plasma drug concentration or amount of drug absorbed.
The United States Pharmacopoeia (USP) also defines IVIVC as "the establishment of a relationship between a biological property, and a parameter derived from a biological property produced from a dosage form, and a physicochemical property of the same dosage form" [5].

Developing the correlation
The basic requirements for developing IVIVC are: • Data obtained from human studies are required for regulatory consideration of the correlation • Two or more drug product formulations with different release rates are developed and their in vitro dissolution profiles generated using an appropriate dissolution method • Usage of same dissolution method for all the formulations, and • Plasma concentration data from a bioavailability study for each of the formulations [6].

Categories of in vitro/in vivo correlations Level A
Level A correlation is usually estimated by a two-stage procedure: deconvolution [7] followed by comparison of the fraction of drug absorbed to the fraction of drug dissolved. Correlation of this type is generally linear. Model Level A correlation is depicted in Figure 1.

Level B
Level B uses the principles of statistical moment analysis. The mean in vitro dissolution time (MDT) is compared either to the mean residence time (MRT) or to the mean in vivo dissolution time ( Figure 2). [7] Level C Level C establishes a single point relationship between a dissolution parameter, for example, t 50% , percent dissolved in 4 h and a pharmacokinetic parameter, e.g., AUC, C max and T max (see Figure 3).

Multiple Level C
A multiple Level C correlation relates one or several pharmacokinetic parameters of interest to the amount of drug dissolved at several time points of the dissolution profile. Various parameters used in IVIVC depends on the level, as depicted in Table1 [7].

IVIVC models
The most basic IVIVC models are expressed as a simple linear equation (Eq 1) between in vivo drug absorption and in vitro drug dissolved (released).
where Y is the in vivo absorbed and X the in vitro drug dissolved, m the slope of the relationship and Cthe intercept. Ideally, m = 1 and C = 0, indicating a linear relationship. Eq 1 may be applied to most formulations with comparable in vitro and in vivo profiles. However, for dosage forms with complicated mechanisms of release, which are usually of longer duration, in vitro release may not be on the same time scale as the in vivo release. Thus, in order to model such data, it is necessary to incorporate time-shifting and time-scaling parameters within the model. [9].

Model validation
Validation is an important tool as it provides the confidence to accept a mathematical model. Here, IVIVC model validation can successfully predict the outcome (in vivo profile) with a given model and test condition (in vitro profile). Internal validation [10] serves the purpose of providing the basis for the acceptability of the model, while external validation [10] is superior and affords greater "confidence" in the model.

Internal validation
Using the IVIVC model, for each formulation, the relevant exposure parameters (C max and AUC) are predicted and compared to the actual (observed) values. The prediction errors are calculated using Eq 3. The criteria set in FDA guidlines on IVIVC are as follows: For C max and AUC, the mean absolute percent prediction error (% PE) should not exceed 10 %, and the prediction error for individual formulations should not exceed 15 % [10].

Prediction
= [(C max observed -C max predicted) / C max observed]*100 ……. ….. (2) Error (%PE) = [(AUC observed -AUC predicted) / AUC observed]*100 ………… (3) External validation [10] For establishing external predictability, the exposure parameters for a new formulation are predicted using its in vitro dissolution profile and the IVIVC model whereby the predicted parameters are compared to the observed parameters. The prediction errors are computed as for the internal validation. For C max and AUC, the prediction error for the external validation formulation should not exceed 10 % [11] A prediction error of 10 to 20 % indicates inconclusive predictability and illustrates the need for further study using additional data sets. For drugs with narrow therapeutic index (TI), external validation is required despite acceptable internal validation, whereas internal validation is usually sufficient for drugs with wide TI.

BIOPHARMACEUTICAL CLASSIFICATION SYSTEM
The biopharmaceutical classification system (BCS) is a scientific framework for classifying a drug substance based on its aqueous solubility and intestinal permeability. [5] • Class I drugs exhibit a high absorption number and a high dissolution number.

Characteristics of drugs in BCS
• Class II drugs have a high absorption number but a low dissolution number.
• Class III drugs exhibit a high variation in the rate and extent of drug absorption. Permeability is rate limiting step for drug absorption.
• Class IV drugs exhibit poor and variable bioavailability.

Estimation
of IVIVC using the biopharmaceutical drug classification system BCS is a fundamental guideline for determining the conditions under which invitro/in-vivo correlations are expected. It is also used as a tool for developing in-vitro dissolution specification [8]. In vitro-in vivo correlation is normally expected for highly permeable drugs or drugs under dissolution rate-limiting conditions. This statement is supported by the biopharmaceutical classification system, which anticipates successful IVIVC for highly permeable drugs [9]. Biopharmaceutical classification system and expected IVIVC for immediate release (IR) and extended release (ER) drug products are summarized in Tables 2 and 3, respectively.
Highlights from some published works on IVIVCs in the context of BCS Classes I -IV are summarized in Table 4 -6, respectively.      Level C IVIVC did not show a perfect correlation, which may be due to single point correlation [40].
In the case of BCS class II drugs (danazol, nifedipine, carbamazepine, piroxicam, nimesulide and griseofulvin), in vitro evaluation includes, flow-through dissolution test, application of Noyes-Whitney equation and comparison of dissolution profiles. For in vivo evaluation deconvolution, Wagner-Nelson and non-compartmental analysis methods were successfully to determine in vivo performance. The in vitro results, compared with the in vivo data by means of level-A followed by level-C, correlated excellently for poorly soluble drugs/products. [41].
For BCS class III drugs (paracetamol, roxatidine, levothyroxine, pseudoephedrine, clonazepam and metformin), Similarity factor (f 2 value) [42] and comparison of dissolution profile were assessed to evaluate in-vitro drug release profile. In vivo evaluation was performed using numerical deconvolution, non-compartmental analysis and Wagner-Nelson methods [43]. On the basis of correlation, level-A, followed by multiple level-C demonstrate reliable and robust estimate of drug product performance.
In-vitro evaluation of BCS Class IV drugs, such as lopinavir and remoxipride, was predicted by comparison of dissolution profile as well as by f-test. For in-vivo evaluation, Wagner-Nelson method and level A correlation exhibited perfect correlation between in-vitro and in-vivo comparison.

APPLICATIONS OF IVIVC IN DRUG DELIVERY
A main objective of developing and evaluating IVIVC is to establish the dissolution test as a surrogate for human bioequivalence studies. However, for the applications outlined below, the adequacy of the in vitro dissolution method to act as a surrogate for in vivo testing should be shown through an IVIVC for which predictability has been established [41].

Early stages of drug delivery technology development: Proof-of-concept
The selection of a drug candidate marks the most crucial stage in the life cycle of drug development. During this stage, exploring the relationship between in vitro and in vivo properties of the drug in animal models provides an idea of the feasibility of the drug delivery system for a given drug.

Future biowaivers
Frequently, drug development requires changes in formulations due to a variety of reasons, such as unexpected problems in stability, development, availability of better materials, better processing results, etc.
Having an established IVIVC can help avoid bioequivalence studies by using the dissolution profile from the changed formulation and subsequently predicting the in vivo concentration-time profile. This predicted profile could act as a surrogate of in vivo bioequivalence study.

IVIVC and parenteral drug delivery
IVIVC can be developed and applied to parenteral dosage forms, such as controlledrelease particulate systems, implants, etc, that are either injected or implanted. However, there are relatively fewer successes in the development of IVIVC for such dosage forms.

Formulation assessment: in vitro dissolution
A suitable dissolution method that is capable of distinguishing the performance of formulations with different release rates in vitro and in vivo is an important tool in product development. IVIVC facilitates the process of such method development [41].

Dissolution specifications
The process of setting dissolution specifications in the presence of an IVIVC starts by obtaining the reference (pivotal clinical batch) dissolution profile. Specifications should optimally be established such that all batches with dissolution profiles between the fastest and slowest batches are bioequivalent and less optimally bioequivalent to the reference batch [42].
Increased development of modified release dosage forms necessitates investigating the broader aspects of in vitro in vivo correlation (IVIVC).

CONCLUDING REMARKS
Based on data from the literature, it is evident that current IVIVC studies have focused more on the development and validation of level A IVIVC which gives more useful information on the relationship between in vitro release and in vivo absorption from dosage form. Levels B and C IVIVCs have been evaluated for several purposes in formulation development, for example, to select the appropriate excipients and optimize the manufacturing processes, for quality control purposes and for characterizing the release patterns of newly formulated immediate release and modified-release products relative to the references. Present regulatory guidelines for IVIVC is only applicable to oral conventional and modified release dosage forms; however, further research is necessary to develop IVIVCs for non-oral products, inhaled medicines and dermatological medicaments. Also, it is possible that the IVIVC can still be explored to provide a greater understanding of the factors influencing clinical quality.