When a small biotech company from Canada trading at less than 1CAD announces plans to list on the Nasdaq, one ought to pay attention, especially when this company is well known for its fiscal conservatism. The recent publication (Geisbert et al.: Postexposure protection of non-human primates...) on the treatment of Ebola infection by Tekmira and collaborators at Boston University and the US Army Medical Research Institute of Infectious Diseases (USAMRIID) in the prestigious journal The Lancet indeed supports that their optimism of good news to come out over the next couple of months is well-founded.
By demonstrating the complete protection of non-human primates, aka monkeys, from an otherwise lethal dose of Ebola virus using the company’s SNALP delivery technology, this paper sets a new standard in the development of anti-hemorrhagic fever virus treatments. More than the potential direct financial value of such a drug in the form of further biodefense funding for the benefit of SNALP technology development and long-term prospects of stockpiling contracts, the real significance of this publication is that it confirms SNALP, developed and owned by Tekmira, to be the industry-leading systemic RNAi delivery technology where successful non-human primate data have become routine, and furthermore extends the reach of this technology from targeting genes in the liver and solid cancers (already a plethora of applications here) to infectious and likely inflammatory diseases, too. Given that Big Pharma is well aware of the potentially revolutionary nature of RNAi Therapeutics and Tekmira’s current key enabling position in the space, it should only be a question of time that a bidding war for Tekmira will commence, if it has not already.
Ebola presents a biosecurity threat due to its deadly nature and the concern that it may be weaponized while no drugs can successfully treat it. Oligonucleotide-based approaches look most promising in the race to develop drugs against Ebola and other hemorrhagic fever viruses. AVI Biopharma in particular has staked a good part of that company’s future on winning biodefense stockpiling contracts from the US government, and as a result their antisense morpholino protein translation inhibitors have historically led the development. Such therapeutics aim to slow down the spread of the virus enough to give the immune system a chance to catch up and clear the virus.
In 2006, AVI and their collaborators around Dr. Bavari from USAMRIID reported results on a combo of morpholino antisense molecules against 3 Ebola genes (L polymerase, VP24 and VP35- targeting 3 different genes to address potential mutational drug resistance) in mice, guinea pigs, and non-human primates (rhesus macaques; Warfield et al.: Gene-specific countermeasures against Ebola virus...). In each case, 1,000 plaque forming units (pfu) of Ebola virus were administered, doses more than sufficient to essentially kill all the animals. In mice, somewhere between 50-500 microgram of drug (about 2-20mg/kg) administered either a little bit before or after the viral infection was enough to effectively protect the animals. In guinea pigs, 10mg of each oligo (~30mg/kg) also protected the animals well. Interestingly, even more protection was observed when the oligos were administered relatively late (up to 4 days) after viral infection compared to administration before viral challenge. While the pharmacologic explanation remains to be worked out, the ability to protect from Ebola relatively late after the infectious event is an important property of an Ebola antiviral.
While mice and guinea pig data are a routine component in the development of Ebola antivirals, success in these models by no means predicts success in primates, including humans, where the rhesus macaque model has become the gold standard. For this reason, 1,000 pfu of the Ebola virus Zaire were injected intramuscularly accompanied by a complex schedule of essentially daily drug injections (12.5mg to 200mg per dose) starting at day 2 before up to 9 days after viral challenge.
Initially, only a single antisense oligo against VP35 was tested which showed most efficacy in rodents. This proved to be ineffective with all 4 animals dying within 7 and 8 days post-infection. However, using the triple combo, 2 out of the 4 drug-treated animals survived the virus with one succumbing to a bacterial infection (= 50% or 75% survival depending on how you look at it). The most common side-effects were thrombocytopenia and an increase in liver enzymes, both of which are consistent with Ebola infection.
Until the Tekmira study, this has been the most advanced published study of an Ebola antiviral drug. One draw-back of the non-human primate studies, however, is that the treatment schedule does not allow for predicting how the triple morpholino combo will perform when given only after the viral challenge. Moreover, protection was not complete.
In unpublished follow-ups, AVI Biopharma and USAMRIID reported apparent advances in the potency of the morpholino chemistry (PMO vs PMO+). When rhesus macaques were treated with the new chemistry for up to 15 days, 75% of animals survived (I assume an n=4) ‘when the treatment period ended’. Clearly, more details are required to ascertain that this represented an improvement over the 2006 study, including survival data beyond the 15 day treatment period.
In light of these results, the SNALP-RNAi study sets a new standard for the efficacy of a prospective Ebola drug. This claim is supported by the fact that the model used in the Tekmira study was essentially identical to the one in the AVI study: rhesus macaques infected with 1,000 pfu of EBOV Zaire and treated with triple oligo combos against the L polymerase, VP24, and VP35 genes (note that any combination of siRNAs can be formulated in one step into SNALPs meaning that it is entirely amenable to rapid response applications). However, in this case the drug (2mg/kg siRNA) was given only after viral challenge, albeit starting already at 30 minutes after the challenge, either daily for 7 days or every other day for 6 days. Moreover, in the daily treatment regimen which is most similar to AVI’s study, all 4 animals receiving the SNALP-siRNA cleared the virus and survived while 2 out of the 3 animals in the alternate-day group survived.
Importantly, this was achieved in the absence of immune stimulation by the SNALP, which was very effectively abrogated by the 2’-o-methylation of the siRNAs. This is notable, because while the 2006 paper on SNALP-siRNA for Ebola in guinea pigs by the same group showed promising efficacy (>60% protection), it was clear that immunostimulation caused by the unmodified siRNAs and amplified by the SNALP was certainly a safety issue then (Geisbert et al.: Postexposure protection of guinea pigs...). Overall, the treatment regimen, 2mg/kg siRNA in SNALP every day for 7 days represents a very stringent test of the safety of SNALP and was very well tolerated, with minor increases in liver enzymes most likely attributable to the viral infection and not the SNALP-siRNA treatment.
It should also be noted that this study employed 1st generation SNALPs which means that the 10 to 100-fold more potent 2nd generation SNALPs as recently reported in Nature Biotech should widen the therapeutic window even more and is important when thinking about safety studies in human volunteers. It is clear, however, that while this study is an eye-opening proof-of-concept of SNALP-RNAi for the treatment of acute viral infections (at least those affecting SNALP-relevant cell types, phagocytic and hepatocytes in this case), any further development (objectives: pharmacology, biomarkers, testing SNALP RNAi at later time-points) towards biodefense stockpiling SNALP-RNAi for Ebola and/or an FDA-approved treatment for accidental needle sticks and sporadic Ebola outbreaks will depend on support by public funding agencies. It is difficult to imagine that this study can be ignored by the US government. The series of head-turning, top-quality SNALP papers in journals like Nature Biotech, Nature, Journal of Clinical Investigations, and The Lancet certainly won’t be ignored by Big Pharma.