The Design and Evaluation of Catalytic MetalloDrugs Targeting HCV IRES RNA: Demonstration of a New Therapeutic Approach

Traditional drug design has been very effective in the development of therapies for a wide variety of disease states but there is a need for new approaches to drug design that will not only be able to tackle new challenges but also complement current approaches. The use of metals in medicine has had some success and allows for the introduction of new properties that are unachievable using only organic compounds but also introduces new challenges that can be addressed by careful design and an understanding of inorganic chemistry. Toward this end, catalytic metallodrugs are being developed for the irreversible inactivation of a therapeutically relevant target. A catalytic metallodrug consists of a metal-binding domain that mediates chemistry and a target recognition domain that provides specificity for the therapeutic target of interest. This approach has a number of advantages including a potential for higher specificity leading to lower doses as well as a unique mechanism of action that will complement current therapies and help combat resistance. Previous work has shown the inactivation of enzymes by irreversible modification of key residues. This approach was then extended to RNA where the backbone is more likely to be susceptible to hydrolytic and oxidative cleavage. Phenylalanine transfer RNA was used as a starting point to look at reactions with RNA and moderate chemistry was observed. This was then further extended to the study of TAR, a therapeutically important RNA found in HIV. Activity was not observed for the degradation of TAR despite the presence of the RNA binding domain, further supporting a double filter mechanism where binding alone is not sufficient for chemistry to occur. A metallopeptide, Cu-GGHYrFK-amide (1-Cu), targeting stem loop IIb of the internal ribosomal entry site (IRES) of the hepatitis C virus (HCV), was tested and found to exhibit good activity both in vitro and in a cellular replicon assay. In the absence of an accepted animal model for hepatitis C infection, this replicon assay is accepted by the FDA as a measure of drug efficacy. A reduction of HCV RNA levels in this replicon assay was confirmed by real-time polymerase chain reaction (RT-PCR). This compound also showed additive to slightly synergic activity when given in combination with the current therapy, recombinant interferon alpha-2b. Derivatives of this peptide were studied to provide insights into the mechanisms of binding and reactivity as well as to enhance properties in vivo . Another set of peptides based on the human La protein was also studied in terms of their interaction with both stem loop IIb and stem loop IV and were shown to have good activity both in vitro and in the same cellular replicon assay. These metallopeptides represent a new class of compounds for the treatment of hepatitis C infection and demonstrate the potential of catalytic metallodrugs as a novel approach to drug design.