Drug-transporter interaction testing in drug discovery and development

The human body consists of several physiological barriers that express a number of membrane transporters. For an orally absorbed drug the intestinal, hepatic, renal and blood-brain barriers are of the greatest importance. The ATP-binding cassette(ABC) transporters that mediate cellular efflux and the solute carrier transporters that mostly mediate cellular uptake are the two superfamilies responsible for membrane transport of vast majority of drugs and drug metabolites. The total number of human transporters in the two superfamilies exceeds 400, and about 40-50 transporters have been characterized for drug transport. The latest Food and Drug Administration guidance focuses on P-glycoprotein, breast cancer resistance protein, organic anion transporting polypeptide 1B1(OATP1B1), OATP1B3, organic cation transporter 2(OCT2), and organic anion transporters 1(OAT1) and OAT3. The European Medicines Agency's shortlist additionally contains the bile salt export pump, OCT1, and the multidrug and toxin extrusion transporters, multidrug and toxin ex-trusion protein 1(MATE1) and MATE2/MATE2 K. A variety of transporter assays are available to test drugtransporter interactions, transporter-mediated drugdrug interactions, and transporter-mediated toxicity. The drug binding site of ABC transporters is accessible from the cytoplasm or the inner leaflet of the plasma membrane. Therefore, vesicular transport assays utilizing inside-out vesicles are commonly used assays, where the directionality of transport results in drugs being transported into the vesicle. Monolayer assays utilizing polarized cells expressing efflux transporters are the test systems suggested by regulatory agencies. However, in some monolayers, uptake transporters must be coexpressed with efflux transporters to assure detectable transport of low passive permeability drugs. For uptake transporters mediating cellular drug uptake, utilization of stable transfectants have been suggested. In vivo animal models complete the testing battery. Some issues, such as in vivo relevance, gender difference, age and ontogeny issues can only be addressed using in vivo models. Transporter specificity is provided by using knock-out or mutant models. Alternatively, chemical knock-outs can be employed. Compensatory changes are less likely when using chemical knockouts. On the other hand, specific inhibitors for some uptake transporters are not available, limiting the options to genetic knock-outs.

Bispecific antibodies in cancer therapy:Target selection and regulatory requirements

In recent years,the development of bispecific antibodies(bsAbs)has been rapid,with many new structures and target combinations being created.The boom in bsAbs has led to the successive issuance of industry guidance for their development in the US and China.However,there is a high degree of similarity in target selection,which could affect the development of diversity in bsAbs.This review presents a classification of various bsAbs for cancer therapy based on structure and target selection and examines the advantages of bsAbs over monoclonal antibodies(mAbs).Through database research,we have identified the preferences of available bsAbs combinations,suggesting rational target selection options and warning of potential wastage of medical resources.We have also compared the US and Chinese guidelines for bsAbs in order to provide a reference for their development.