Pharmacokinetic Properties and Applications of Drug Carriers

To date, poly[N-(2-hydroxypropyl)methacrylamide] (PHPMA) is the most advanced example of a synthetic polymer used as a drug carrier in both basic research and clinical applications reported by Duncan12 and Kocheck13. The biocompatibility of this polymer has been well characterized,14 and it was found to have an inert character when it was injected in the blood stream. , A PHPMA copolymer conjugated with doxorubicin as an anti-tumor agent linked via a peptidyl linker

(Gly-Phe-Leu-Gly) was developed. The PHPMA copolymer conjugated with doxorubicin is stable in plasma17 and has been shown to be concentrated within solid tumor models.18'19 It is then cleaved intracellulary by lysosomal cysteine proteinases,20 thereby allowing intratumoral drug release. Preclinical investigations have shown that a PHPMA copolymer conjugated with doxorubicin has radically different pharmacokinetics compared to free doxorubicin with a distribution plasma half-life that is increased from 5 min to 1 h.21 The stable peptidyl linker also ensures that little or no free doxorubicin is released into the circulation following intravenous injection, increasing the therapeutic index of the conjugate. Besides the prolonged circulation in blood and the high solid tumor accumulation, it is also possible to attach targeting moieties such as antibody22 and saccharides23 to PHPMA. Another polymer conjugate, poly(ethylene glycol) (PEG), is the most basic and widely used polymer. Stylene maleic anhydride conjugated with the anti-tumor protein, neocarzinostatin (SMANCS), has been studied and is already used for the treatment of hepatocellular carcinoma.24 Furthermore, by attaching homing devices as a cell-specific uptake enhancer, the biodistribution could be altered and cellular targeting of drugs conjugated with polymer could be achieved.

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