After entering the blood, the drug will be distributed along with the blood flow into various tissues and organs of the body. Generally, the drug is first distributed into the tissues with a high blood flow rate, such as the liver and kidney. It then enters the tissues with a low blood flow rate, such as muscle and fat. The tissue distribution of drugs depends on physiological factors such as tissue blood flow rate and physiological barriers and drug properties such as lipophilicity, plasma protein binding rate, and tissue affinity.
Plasma protein binding is an important parameter to characterize the drug distribution process. In the blood, most small molecule drugs are reversibly bound to plasma proteins. In the absence of active transport processes, only free drugs can penetrate the biological membrane into the corresponding tissues, exert pharmacological effects or undergo metabolism and excretion. When the distribution process reaches equilibrium, the bound drugs in the plasma and tissues are in dynamic equilibrium with free drugs. Plasma protein binding may cause multiple effects, such as slowing down metabolism, reducing clearance, prolonging elimination half-life, and limiting drug distribution into target tissues. Plasma protein binding is of great significance in building the PK/PD model, predicting drug-drug interactions, evaluating drug toxicity, predicting human PK parameters and dose.
Our drug distribution and protein binding study platform utilizes three models to investigate plasma protein binding: equilibrium dialysis, ultracentrifugation, and ultrafiltration. The data obtained by these methods are in agreement with those in the literature. In addition to plasma protein binding, this platform can also carry out the study of drug binding to human serum albumin and α1-acid glycoprotein. Drug binding studies can be conducted to other biological matrices, such as tissue homogenate, hepatocytes, liver microsomes, whole blood, and drug distribution studies such as drug partition ratio in blood and plasma.
We selected a series of drugs with high, medium, and low protein binding rates, respectively, and performed a plasma protein binding assay using in-house methods. The results obtained correlate well with the literature results.
Figure 1. Correlation of free fraction of commercial drugs in human plasma between literature data (abscissa)  and in-house data (ordinate). Each data point represents the mean of three data. A. equilibrium dialysis (HTD) method; B. equilibrium dialysis (RED) method; C. ultracentrifugation; D. ultrafiltration; The following drugs were analyzed: Amiodarone hydrochloride, Atenolol, Chlorpromazine Hydrochloride, Clozapine, Diclofenac sodium salt, Imipramine, Metoprolol, Montelukast, Propafenone hydrochloride, Quinidine, Saquinavir, Topotecan hydrochloride, Verapamil hydrochloride, Ceftriaxone, Ibuprofen, and Cyclosporine.