Polymer-Drug Conjugates for Anticancer Drug Delivery.

Polymer-drug conjugates (PDCs) are drug delivery systems where one or more drug(s) are covalently attached to the functional groups of the polymer directly or through a spacer. Several anticancer drugs that have been used to synthesize PDCs are currently under clinical trials. PDCs have shown enhanced tumor accumulation, increased therapeutic index, and prolonged circulation, accompanied by a sustained release of the bound drug. Distinct cell uptake mechanisms make PDCs less sensitive to efflux pumps associated with the development of multi-drug resistance. However, the effectiveness of PDCs as a delivery system primarily depends on the drug, polymer, type of linkage, and presence of targeting groups. Due to the availability of different functional groups and spacers, it is possible to control drug release as well as multi-functionalize PDCs, thereby increasing their versatility as drug carriers. Furthermore, active tumor uptake may be achieved by using the concept of drug targeting. However, functionalization alters the in vivo behavior of the polymer, signifying the evaluation of safety and effectiveness of PDCs. Several PDCs are currently being tested in different phases of clinical trials. This review focuses on critical aspects in the design of PDCs when used in cancer drug delivery.