Over the last several years, injectable drugs have been a growing area for the treatment of various therapeutic conditions and they are projected to comprise an even larger proportion among the drugs that will be available in the years to come. The injectable drugs are administered by various routes such as intramuscular (IM), intravenous (IV), subcutaneous (SC) and others, however, the majority of these drugs are administered subcutaneously. Even though subcutaneous delivery has been utilized for a number of years, very little is known about the processes governing the absorption of macromolecules from the interstitial space; and the resulting impact of these processes on the bioavailability (BA) and pharmacokinetic (PK) profiles. Also, there is no established In vitro - In vivo correlation (IVIVC) for subcutaneously administered immediate release (IR) peptide based drugs in a biorelevant manner. The contribution of IVIVC in drug development of orally administered drugs is very well known. For oral drugs, the in vivo process of drug absorption is often rate limited by the rate at which drug dissolves in the gastrointestinal tract. This can be simulated by measuring the rate of dissolution in an in vitro apparatus, which can be correlated with the in vivo absorption rate to produce an IVIVC. This research program involved efforts to develop biorelevant IVIVC methods and model for subcutaneously administered peptide based drugs. The in vivo component of this Program involves the use of clinical data from a bioequivalence (BE) study of Iplex™ [(IGF-I (Insulin like growth factor-I)/IGFBP-3 (Insulin like growth factor binding protein-3)], administered subcutaneously, that was conducted at the Center for Drug Studies (CDS), VCU School of Pharmacy in the year 2005 (Barr et. al., 2005). The PK parameters for Increlex™ (IGF-I) are calculated from the clinical data obtained from another study (Rabkin et. al., 1996). Literature research and molecular modeling research formed the basis of our hypotheses that unbound and bound IGF-I are absorbed from the blood capillaries and lymphatic capillaries respectively and that simulation of these physiologic variables is possible with the use of the modified Hanson Microette® device. The Hanson Microette® device is a vertical diffusion cell system that has been modified to simulate the pores in the capillaries with the use of a synthetic membrane. The flow and composition of circulatory fluid was simulated by the use of modified Hanks balanced salts solution (HBSS). A validated RP-HPLC (reversed-phase high performance liquid chromatography) method has been used for the analysis of IGF-1/IGFBP-3 in the in vitro samples. The in vitro permeation/release results gave the in vitro component to conduct IVIVC analysis. The General Electric (GE) healthcare sourced polycarbonate nucleopore track etched membranes were the only set of membranes that resulted in significant permeation in the in vitro experiments. IVIVC results demonstrated high inter and intra-membrane variability for the membranes (available from today’s technology) that were used to simulate the in vivo membrane characteristics. Currently, there are no validated biorelevant IVIVC methods for SC formulations. The methods described here are the basis for future in vitro method development that will be of significant value for (a) predicting the in vivo performance of SC formulations based on the in vitro data, and (b) provide a reproducible in vitro method as the basis of developing an IVIVC for other subcutaneously administered drugs. This will provide an important tool for both development and regulation of this growing class of drugs.