By Derek Lowe
We have two pieces of news about the Gamaleya Institute’s “Sputnik-V” vaccine today. Neither of them are going to be enjoyable to go into.
First off, many may have heard that the Brazilian regulatory authorities had a hearing yesterday to see if this vaccine would be approved for use there. They have turned it down, for several reasons. (Update: here are their slides, in Portuguese). Among these are questions about the manufacturing and scale-up processes, which I have to say have not been very well documented for this vaccine. Readers may recall the reports from Slovakia about the authorities there getting what appeared to be completely different formulations of the vaccine all shipped together, so there is some room for clarity about how these processes are controlled. But the bigger news was that Anvisa, the Brazilian drug agency, said that every single lot of the Ad5 Gamaleya shot that they have data on appears to still have replication-competent adenovirus in it. Let’s back up for a second to appreciate what the means, for readers who don’t do this stuff for a living. The next few paragraphs are background; skip if you like.
The adenovirus vector vaccines (all of them so far in the pandemic – AZ/Oxford, J&J, Gamaleya, CanSino) are made by removing most of the adenovirus DNA instructions from some form of the virus, and inserting DNA to make coronavirus antigens instead. Oxford has a chimpanzee adenovirus, J&J has been using the Ad26 strain, CanSino has the Ad5 adenovirus, and the Gamaleya vaccine is one shot of Ad26 followed by a shot of Ad5. But all of them carry the DNA to make the coronavirus Spike protein (some of them in its native state, others with stabilizing amino acid mutations). And all of them have had key parts of their original genome removed to make them unable to replicate in the body (deleting a gene called E1 is the standard way to do this).
That means that when you’re injected with such a vector vaccine, each viral particle is a one-shot deal. It infects a cell in your body and instructs it to make Spike protein (thereby setting off your immune response when that foreign protein gets presented), and that’s it. A real wild-type virus would of course make the whole suite of viral proteins, which would be assembled into countless new virus particles. These would then be released when the cell finally breaks apart and dies from the overload. Now, there have been debates over the years about whether you’d get a more effective vaccine that way, with a “replication-competent” adenovirus, but generally it’s believed that you can do fine with the “replication-incompetent” ones, which let you *not* give your patients a new viral infection at the same time.
Adenoviruses are everywhere. What happens when you’re infected with a wild-type variety? Generally you get respiratory infections that vary according to the person. With Ad5 and Ad26 they’re generally mild, sometimes unnoticeable, but in some people there can be serious trouble, which is another reason to avoid giving them replicating virus. The Ad5 variety has infected a solid proportion of the entire human race, as far as we can tell, which is one reason why you see people moving to less-common platforms like Ad26 or adenoviruses from other primate species entirely (as with Oxford/AZ). It’s believed that if you already have antibodies and T-cells primed against the Ad5 vector itself (for example) that delivery of its payload will thus be impaired, leading to a less-effective vaccination. This also makes you wonder about diminished efficacy of booster-shot regimens with such vectors, no matter what strain you start with, and what happens if you want to get vaccinated a few years later against a completely different pathogen whose vaccine uses a viral vector you’ve already been exposed to. For now, it looks like the booster-shot idea can work, although the second shot is surely chewed up more by the immune response. The second concern is still an open question, as far as I know. Presumably both of these would be even bigger concerns with a replication-competent vaccine, because you’re hitting the patients with an even stronger viral challenge.
OK, now we need to talk about how you make big piles of virus if you’ve kept them from replicating. That’s an interesting question that has a slick solution: you’re going to be using human cells (often the HEK293 line) to expand your virus production, and what you do is engineer those human cells so that they make the missing E1 protein that the virus needs to replicate. So as long as you’re growing up virus in these engineered cells, you’ll make more, but if they infect normal human cells that haven’t been jiggered to make a key viral protein (as in when you inject them as a vaccine) they’ll stall out on replication immediately. Problem solved!
Mostly. There are still places where this can go wrong. Double-stranded DNA breaks, which can happen more or less randomly, are generally repaired by processes called “homologous recombination” and “nonhomologous end joining”, and these can lead to mix-and-match behavior between DNA from different sources. This process can be deliberately harnessed for gene editing – that’s what the classic CRISPR enzyme Cas9 does – but it can also be a source of trouble in a system like this one. There is a chance that the occasional viral particle might be able to regain the DNA sequence for the E1 protein by picking it up from the human-cell background. If that goes right (well, wrong), then that will turn it back into a replicating virus, and that’s just what it will do in your cell culture tanks.
This problem has been recognized for many years, of course, and no one’s forgotten about it. There are assays to screen for this sort of thing, and every time work gets going on a proposed new viral vector, updated assays are developed to screen that one, too. You can dive into these references for details on how to engineer both your viral vectors and your human cell lines to cut down on the chances of this happening as well. This is an accepted part of the vector vaccine production process.
Which is why the news that the Sputnik vaccine contains replicating adenovirus was surprising and unwelcome. As mentioned, this is probably not going to cause big problems in its vaccinated population, but it’s a completely unnecessary risk. And if such a vaccine is going into tens of millions of people (or more), it seems certain that there will be some people harmed by this avoidable problem. If you’re going to make a replication-competent vaccine, make one and run the clinical trials with it, and if you’re asking for regulatory approval for a replication-incompetent one you shouldn’t show up with an undefined mixture of replicating and non-replicating viral particles instead. This sort of thing calls into question the entire manufacturing and quality control process, and I can see why the Brazilian regulators are concerned.
The response from the Sputnik V camp has not been good. The official Twitter account has accused Anvisa of having “invented fake news” about the vaccine, when what you’d hope to see is more of the good ol’ “We stand by our manufacturing process, but take these concerns seriously and are working with the authorities to resolve this question” sort of thing. But no, it’s all for “political reasons”. Their official statement is no more conciliatory. A tip for the vaccine’s manufacturers: don’t immediately start accusing your critics of bad faith, especially when they are the regulatory authorities. Step up and act like responsible drug developers: address the issues directly, with transparency, and work to find a solution. Throwing fits on Twitter is not the answer. But that brings us to the next post, coming up shortly. . .
https://blogs.sciencemag.org/pipeline/archives/2021/04/28/brazil-rejects-the-gamaleya-vaccine
