Thursday, June 14, 2007

Benitec and City of Hope Start RNA-targeted HIV Phase I Clinical Trial

After some delay, Benitec and their collaborators from the City of Hope finally announced that they obtained regulatory approval to start phase I clinical trials for an RNA-based HIV gene therapy antiviral. The delay was caused by Benitec’s uncertain corporate future and difficulties generating sufficient amounts of clinical-grade lentiviral vectors, but scientifically the risk-(potential) benefit profile of these studies are promising.

This is the 8th RNAi clinical program and the second involving DNA-directed shRNAs. The gene therapy agent is a lentiviral vector expressing 3 different RNA molecules, each designed to interfere with a different aspect of HIV replication, each through a distinct mechanism of action. The plan is to immunise CD34+ hematopoietic progenitor cells and thereby all their progenies, including T-cells, with such vectors ex vivo and then re-administer the cells to the patients.

The RNAi portion is a U6-driven hairpin RNA targeting the tat/rev mRNA by RNAi. A nucleolar-localised TAR decoy RNA, also under the direction of a U6 promoter, is designed to mimic the HIV TAR element, thereby diverting TAR-binding factors by mimicry. Finally, a ribozyme against the CCR5 mRNA, a co-receptor for HIV infection, complements the 3-pronged approach. This approach is inspired by the current HAART HIV treatment paradigm where a cocktail of antiretroviral drugs has proven to be highly effective in suppressing HIV replication with only slow development of drug resistance.

In fact, the RNA-based vector which has been shown in tissue-culture experiments to be considerably active in inhibiting HIV replication may be synergistic with present therapies due to their unique mechanism of action, although non RNA-based anti-CCR5 treatments are currently developed by other companies as well. While Benitec used to pursue a triple RNAi approach for the treatment of HCV (a program now owned by Tacere), the present strategy may be advantageous since it is known that co-transcribed shRNAs or co-transfected siRNAs may compete with each other for cellular RNAi factors.

Ultimately, I expect that long-term expression of ideally all 3 RNAs will be important to confer a survival advantage onto the lentivirally transduced CD34+-derived cells. However, even if expression should be silenced eventually, the treatment is likely to give AIDS patients some reprieve. The study population will be 5 AIDS-related leukaemia patients from which CD34+ will be enriched from blood by apheresis, genetically modified, and then returned to the donor patient.

Given that this is a gene therapy with a vector that hasn’t been used in the clinic before, it is expected that this therapy will be used in AIDS/Lymphoma patients who are no more responsive to conventional treatments. Nevertheless, expect to hear results from this promising phase I trial within a year.

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By Dirk Haussecker. All rights reserved.

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