I’ve been reminded a number of times by the staunch Benitec-supporters here that Texas-based biotech company Gradalis has been moving a ddRNAi-enhanced cancer vaccine candidate (‘FANG’) aggressively through clinical development. Virtually out of nowhere, Gradalis initiated clinical trials two years ago and there are now two active phase II trials, one in ovarian cancer and one for advanced melanoma. A peer-reviewed publication on the phase I trial was also just published (Senzer et al.) arguably making FANG the lead RNAi candidate in oncology.
The phase I study involved over 40 patients with advanced solid tumors and demonstrated the safety and logistic feasibility of the approach. Although evidence of suggestive of efficacy was presented such as a clear correlation between an immune response and survival, it would be premature to conclude anything with regard to efficacy. Having now followed a number of cancer vaccines, most of which have eventually failed, it seems to me that correlations such as this could be just as well as a reflection of the fact that those with more responsive immune systems will do better anyway.
‘FANG’ comprises of plasmid DNA from which a single RNA polymerase II promoter drives the expression of an upstream GM-CSF open-reading-frame followed by a pair of downstream RNAi hairpins. This plasmid is introduced by electroporation in a petri dish into the patients’ own cancer cells which have been obtained from a tumor resection. After allowing some time for the expression of the transgenes, the cells are irradiated so as to kill off their proliferative potential and are then re-introduced like many other vaccines by intradermal injection.
The GM-CSF component, wildly popular in the cancer vaccine field and also part of Dendreon’s famous prostate cancer vaccine PROVENGE, is supposed to serve as an attractant, proliferation and maturation factor for dendritic cells which are supposed to ingest, present and thereby stimulate an immune response against the antigens unique to a tumor; the pair of ‘bifunctional’ hairpins meanwhile both target furin which is thought to be an important protease for the maturation of the various isoforms of TGF-beta, a well-known immunosuppressant often overexpressed in cancer.
‘Bifunctional’ here means that one hairpin is perfectly matched and therefore mostly relies on the so-called Ago2/cleavage-dependent mode of RISC activation, whereas the other hairpin contains a central bulge due to mismatching changes introduced in the passenger strand arm of the hairpin thus relying on the non-cleavage pathway of RISC activation which can be facilitated by all four human Argonautes (both predicted to yield the identical guide strand). This strategy of distributing the RNAi between the various Argonaute proteins is certainly an interesting idea, but I’m not sure whether even Gradalis knows what consequences of this is both in terms of efficacy and safety.
A general lack of detailed molecular mechanistic studies is probably my biggest concern with this candidate and when thinking about Gradalis in general. It also at least partly explains why FANG has been moving so rapidly through the clinic. I find it particularly troubling that I have seen no detailed studies by Gradalis looking at the relationship between furin knockdown and TGFbeta inhibition which is key for Gradalis' strategy. This already has caused difficulties in interpreting some of the phase I data where possible assay problems complicated reconciling apparently only modest reductions in furin with much more pronounced down-regulations of TGFbeta. This not only makes it more difficult to make the right development decisions, but also when it comes to finding a partner for the program. On the other hand, you could argue that a cancer vaccine candidate involving both GM-CSF expression and TGFbeta inhibition already has a good chance at succeeding, and sweating out the technical details would only cause delays without making us much the wiser.
As I had mentioned, the Benitec supporters are following Gradalis’ development with much interest as such an advanced ddRNAi candidate may be a prime licensing opportunity for Benitec which controls an important part of the ddRNAi patent landscape. I’ve certainly looked at the hairpin structures involved in light of Benitec’s patent claims (esp. the ‘099 Graham patents) and there is a good chance that Gradalis ought to take a license as it further monetizes this candidate, although their structures may give them a bit of wiggle room.
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