In-Vitro In-Vivo Correlation

INTRODUCTION
Formulation development and optimization is an ongoing process in the design, manufacture and marketing of any therapeutic agent. Formulation optimization may require altering formulation composition, manufacturing, equipment and batch sizes. In the past when these types of changes are applied to a formulation, bioavailability studies would also have to be performed in many instances to ensure that the “new” formulation displayed statistically similar in-vivo behaviour as the “old” formulation. Of course this requirement delayed the marketing of the new formulation and added time and cost to the process of formulation optimization.
Recently a regulatory guidance was developed to minimize the need for additional bioavailability studies as part of the formulation design. This guidance referred to as the, In Vitro/In Vivo Correlation Guidance was developed by the Food and Drug Administration and was based on scientifically sound research. The main objective of developing and evaluating an in-vitro in-vivo correlation (IVIVC) is to enable the dissolution test to serve as a surrogate for in vivo bioavailability studies. This may reduce the number of bioequivalence studies required for approval as well as during scale-up and post-approval changes (SUPAC).
A valid IVIVC will allow for dissolution testing for subsequent formulation changes which take place as a function of product optimization without the need for additional bioavailability/bioequivalency studies.
An IVIVC has been defined by the Food and Drug Administration (FDA) as “a predictive mathematical model describing the relationship between an in-vitro property of a dosage form and an in-vivo response”. 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, or a parameter derived from a biological property produced from a dosage form, and a physicochemical property of the same dosage form”. Typically, the parameter derived from the biological property is AUC or Cmax, while the physicochemical property is the in vitro dissolution profile. A linear relationship with slope of unity, if possible, is preferred, as the dissolution profile is a representative of the absorption profile.
Since, IVIVCs are basically mathematical relationships, non-linear correlations may also be appropriate. IVIVC plays an important role in product development in that it:

• Serves as a surrogate of in vivo and assists in supporting biowaivers; 
• Supports and/or validates the use of dissolution methods and specifications; and
• Assists in quality control during manufacturing and selecting appropriate formulations .

The first and main role of establishing IVIVC is to use dissolution test as a surrogate for human studies. The benefit of this is to minimize the number of bioequivalence studies performed during the initial approval process and during SUPAC. Additional advantage of an IVIVC is to assist in validating or setting dissolution specifications. This is because the IVIVC includes in-vivo relevance to in vitro dissolution specification. In other words, dissolution specifications are set based on the performance of the biobatch in-vivo.
A validated IVIVC is of significant benefit for pharmaceutical manufacturers due to minimizing the time and cost invested in additional bioavailability studies. In addition, IVIVC is normally expected for highly permeable drugs or drugs under dissolution rate-limiting conditions. The use of IVIVC, however, is limited to a certain drug product. It can be used only on that particular formulation. The IVIVC cannot be used across the products, especially drug product with different release mechanisms. 

Levels of IVIVC

IVIVC studies have been reported for various drugs. The studies were conducted both in animal, such as rat, rabbit, and dog and human.

Level A correlation

Current IVIVC studies have focused on the development and validation of a level A correlation. It is a point-to-point relationship between drug release in vitro and in vivo. Although, a concern of non-linear correlation has been addressed, no formal guidance on the non-linear IVIVC has been established. It is a predictive mathematical model for the relationship between the entire in vitro dissolution/release time course and the entire in vivo response time course, e.g. the time course of plasma drug concentration or amount of drug absorbed.

Level B correlation

The level B is a correlation which compares the mean in-vivo dissolution to the mean in-vitro dissolution. It is predictive mathematical model for the relationship between summary parameters that characterize the in vitro and in vivo time courses, e.g., models that relate the mean in vitro dissolution time to the mean in vivo dissolution time.

Level C correlation

Level C correlation describes a relationship between the amount of drug dissolved at one time point and one pharmacokinetic parameter. Level C is also considered the lowest level of correlation. It is a predictive mathematical model of the relationship between the amount dissolved in vitro at a particular time (or the time required for in vitro dissolution of a fixed percent of the dose, e.g. T %) and a summary parameter that characterizes the in vivo time course (e.g., Cmax or AUC). E.g. correlation between Cmax and percent dissolved at 15 min.
Level B and C IVIVCs have been developed for several purposes in formulation development, for example, for selecting the appropriate excipients and optimizing manufacturing processes, for quality control purposes, and for characterizing the release patterns of a newly formulated immediate release (IR) and modified release (MR) products relative to the reference. 

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. Multiple Level C correlations can be as useful as Level A correlations.
Biopharmaceutics Classification System AND IVIVC
Biopharmaceutics Classification System (BCS) is a fundamental guideline for determining the conditions under which in-vitro in-vivo correlations are expected. It is also used as a tool for developing the in-vitro dissolution specification.  The classification is associated with drug dissolution and absorption model, which identifies the key parameters controlling drug absorption as a set of dimensionless numbers: the Absorption number, the Dissolution number and the Dose number.

1. The Absorption number is the ratio of the mean residence time to the absorption time.
2. The Dissolution number is a ratio of mean residence time to mean dissolution time.
3. The Dose number is the mass divided by an uptake volume of 250 ml and the drug’s solubility.

The mean residence time here is the average of the residence time in the stomach, small intestine and the colon.
The fraction of dose absorbed then can be predicted based on these three parameters. For example, Absorption number 10 means that the permeation across the intestinal membrane is 10 times faster than the transit through the small intestine indicating 100% drug absorbed.
Class I drugs such as metoprolol exhibit a high absorption number and a high dissolution number. The rate-limiting step to drug absorption is drug dissolution or gastric emptying rate if dissolution is very rapid.
Class II drugs such as phenytoin have a high absorption number but a low dissolution number. In-vivo drug dissolution is then a rate-limiting step for absorption (except at very high Dose number). The absorption for Class II drugs is usually slower than Class I and occurs over a longer period of time. IVIVC is usually expected for Class I and Class II drugs.
For Class III drugs, permeability is the rate-controlling drug absorption. Furthermore, Class III drugs exhibit a high variability of rate and extent of drug absorbed. Since the dissolution is rapid, the variation is due to alteration of GI physiological properties and membrane permeation rather than dosage form factors.
Class IV drugs are low solubility and low permeability drugs. Drugs that fall in this class exhibit a lot of problems for effective oral administration.
 

BCS and expected IVIVC for Immediate Release (IR) Drug Products
Class	S	P	IVIVC
I	High	High	Correlation if dissolution is rate limiting step
II	Low	High	IVIVC expected
III	High	Low	Little or no IVIVC
IV	Low	Low	Little or no IVIVC

S = solubility; P = permeability
 

BCS for Extended Release (ER) Drug Products
Class	S	P	IVIVC
IA	High & Site Independent	High & Site Independent	IVIVC Level A expected
IB	High & Site Independent	Dependent on site & Narrow Absorption Window	IVIVC Level C expected
IIA	Low & Site Independent	High & Site Independent	IVIVC Level A expected
IIB	Low & Site Independent	Dependent on Site & Narrow Absorption Window	Little or no IVIVC
VA:  Acidic	Variable	Variable	Little or no IVIVC
VB:  Basic	Variable	Variable	IVIVC Level A expected

S = solubility; P = permeability