Antisense strategies for redirection of drug metabolism: using Paclitaxel as a model

The concept of metabolic redirection involves shifting the metabolism of a substrate away from one biochemical pathway to another. There are numerous instances in biological systems of a single substrate being metabolized by more than one enzyme with differing rates and activities, resulting in substantially different products. While an extremely large number of compounds are substrates for these enzymes, the types of reactions catalyzed are finite. This chapter focuses on Taxol(R) (paclitaxel; Fig. 1), an important antineoplastic agent that is used extensively in treating several types of malignancies including ovarian, breast, and lung, as well as melanomas.Paclitaxel, which was originally isolated from the bark of pacific yew Taxus brevifolia, causes promotion and excessive stabilization of the microtubule polymer during cell division. It is generally accepted to be metabolized in the human liver by cytochromes P450 (CYP) 3A4 and 2C8. Although both major metabolites exhibit the excessive microtubule stabilizing properties of the parent compound, the metabolism promotes their excretion in bile in addition to reducing their potency and efficacy. It is reported that the in vitro cytotoxic activity of the CYP3A4 metabolite in cultures of leukemia cells is 10-fold lower than the parent compound. Furthermore, it has been reported that administration of paclitaxel causes an induction in the specific content of CYP3A4 in the liver, thereby promoting its own metabolism. This scenario presents an opportunity to improve the bioavailability and reduce clearance of paclitaxel through inhibition of CYP3A4.The studies presented in this chapter were conducted in rats and utilize the antisense approach to inhibit CYP3A2, which is the rat ortholog of human CYP3A4. Animals were pretreated with AVI-4472, a phosphorodiamidate morpholino antisense oligomer targeted to the CYP3A2 mRNA, and changes in the bioavailability of paclitaxel were examined.