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John Schiller (NCI at NIH) 1: Human Papillomavirus (HPV) Vaccines to Prevent Cancer

August 25, 2019


Well, hello. My name is John Schiller. I work in the Intramural Program of the National Institutes of Health. And today I’ll be giving a lecture on HPV vaccines to prevent cancer. And what I’ll be covering is initially the basic biology of HPV and its association with cancer. And then talk about the development of the vaccines to prevent the cancers that are caused by HPV. And then, in the last third, talk about some of the implementation issues that have arisen with these vaccines, and what we’ve been doing to try to overcome those implementation issues. So, first of all, a little basic molecular biology. So, HPVs all have 8-kilobase double-stranded genomes. And they are divided into two regions. One is called the early region, which encodes factors involved in the replication and transcription of the viral genome, and setting up the cells for viral replication. There are two genes in the early region, E6 and E7, which are selectively retained and expressed in cancers. E6 binds the tumor suppressor gene p53, and E7 binds the tumor suppressor Rb. The other region of the genome is what’s called the late region… region, which I’ll mainly be talking about today, which encodes two proteins: the major virion protein, L1, and the minor virion protein, L2. So, the structure of the virion is depicted on this slide. It’s a non-enveloped virion, about 55 nanometers with a regular icosahedral array composed of 72 pentamers of the L1 major capsid protein and up to 72 copies of the minor capsid protein, L2. And in the middle, not shown here, is the DNA genome, which is histone-bound. Now, the HPVs have a very unusual life cycle. The life cycle takes place entirely within the stratified squamous epithelia. And we believe that this type of life cycle has evolved primarily to escape immune [surveillance]. So, this is a tricky virus. What happens is it initially has to bind the basement membrane — and I’ll talk about that in my second lecture — which then initiates the infectious process of basal keratinocytes. Now, once it infects these lower cells, it expresses very low levels of its virul… of its proteins, just the early proteins, and has autonomous replication at relatively low copy number. But, relying on signals involved in terminal differentiation of the epithelium, there is an induction of over-replication of the genome and the late gene expression, leading to virion assembly and release. And so, by making these virions and exposing them only to the external surface, it essentially evades the immune system, keeps the immune system ignorant of these very immunogenic particles, until quite a delayed period. So, the reason we want to develop HPV vaccines is that HPVs cause cancers, a lot of cancers. It’s been estimated that approximately 5% of all cancers are caused by HPV infection. And the worldwide burden is shown on this slide. And it’s important to note that cervical cancer represents the majority of the HPV-associated cancers worldwide, about 500,000 cases and 250,000 deaths. It’s also important to note that of the 12 or 15 types that are considered oncogenic, two types, 16 and 18, account for 70% of cervical cancer. And they actually account for approximately 90% of the other HPV-associated cancers, which are fewer in number and a lower attributable fraction. Essentially 100% percent of cervical cancers are believed to be caused initially by an HPV infection. Now, in the US, of the attributable cancers that are caused by HPV, the proportions vary somewhat differently than worldwide. And as you can see, cervical cancer doesn’t dominate as much. And there are more male cancers, shown in in gray, than there are worldwide. And the reason for this is two-fold. One is we have PAP screening, which decreases cancer… cervical cancer by approximately 80% in the United States. And secondly, there’s been what’s considered an epidemic of oropharyngeal cancer in the last several decades. It’s essentially doubled. And many of these cancers — as shown in gray, again — are caused in males. Now, HPVs, and the oncogenic HPVs, actually infect many sites in both the male and female genital… genitalia. So for instance, there’s many infections in the vaginal and vulvar regions of the women, and the penile skin of men. But the reason we believe that HPV predominates in cervical cancer is because there’s a place in the anatomical regions of the cervix called the transformation zone that’s particularly susceptible to carcinogenic progression. And this is the region where single columnar epithelia meets stratified squamous epithelia. And quite frankly, we really don’t understand why this particular site is so prone to HPV-induced carcinogenesis. But this incidentally is also the site in which most of the cancers in anal cancer arise as well. Now, this shows the timeline of [HPV] infections in relation to cervical cancer. And what you can see is that the rates of cervical HPV infection rise very rapidly in young adults, and the lifetime incidence of genital HPV infection is considered to be over 80%. So, having a lifetime HPV infection is almost synonymous with being sexually active. But most of these infections go away spontaneously and do not really pose risk for cancer. You need a persistent infection to get cancer. But about a decade later, you see a peak in precancerous lesions. And then about another decade later, you can see that you get a peak in cancer incidence. So, really, persistent infection with oncogenic HPV is a critical risk factor for progression to precancer and cancer. And because HPV infections are so common, they’re rapidly acquired after sexual debut. And this is just data from the US and UK that shows, quite strikingly, that within two years of initiating sexual activity almost half of the young women have had an HPV infection. Now, it’s important to point out that current PAP screening is quite effective, but it’s what we call secondary prevention, which targets adult women who already have their infections and have been diagnosed with a cervical abnormality for further follow up and eventually ablative treatment. And what we’re talking about with the vaccines, now, is what we call primary prevention, which can target both adolescent girls and boys, and eliminate the initiating event and prevent all this downstream follow-up that’s required in treatment for secondary prevention. So, as I mentioned, the real viruses have three components. They’ve got L1 pentamers. They’ve got L2 monomers. And then the HPV genome. But through molecular biology tricks, we can express just L1 and L2 alone, and generate what are called virus-like particles. Now, in animal models, we were able to determine that L1 contains most of the determinants to which neutralizing antibodies can bind. And so that the current vaccines are made up of what we call L1-only VLPs, virus-like particles. So, they’re essentially made up of 360 copies of a single protein, this L1. And therefore they’re considered to be subunit vaccines because they’re non-infectious and non-oncogenic, because they’re just made up of this single protein. And again, we’ve shown in animal studies that I won’t show that VLPs induce extremely high titers of neutralizing antibodies that are at least a log higher than what is seen after a natural infection. So, based upon the promising preclinical data, three distinct HPV L1 VLP vaccines have been commercialized. And they’re made by two manufacturers: GlaxoSmithKline and Merck. And they vary in valency, the number of types included; the adjuvant, which is the immune stimulatory component in the vaccine; and the production system in which the VLPs are made. So, Cervarix, which is made by GlaxoSmithKline, is bivalent. It contains the two types, 16 and 18, which cause 70% of cervical cancer. This immune stimulatory product that is used is a proprietary one. It’s called AS04. It consists of a simple aluminum salt, which would be the standard adjuvant, plus what’s called monophosphoryl lipid A, which is a TLR agonist that further stimulates the immune response. And it’s the first example where the FDA has approved this type of immune stimulator for a prophylactic vaccine. Gardasil is quadrivalent. It contains the same two high-risk HPV types, 16 and 18. In addition, it contains HPV 6 and 11, which cause about 90% of genital warts. It doesn’t cause cancers, but is responsible for genital warts. It contains a much more common aluminum salts vaccine. Gardasil-9, which has been more recently developed and is actually the vaccine that’s now most available in the United States, is nonavalent. So, in addition to the two types in Cervarix, it contains the five next types that are most often seen in cancer. But other than that, it’s very much formulated the same way. Now, the production system also varies. So, Cervarix is made in insect cells, which was the production system we initially used in our preclinical studies in a proof-of-principle evaluation, whereas Gardasil is made in Saccharomyces cerevisiae yeast, common bread yeast. So, the vaccines are administered by intramuscular vaccination, generally at one and either one or two months… at zero, one, or two months, and then six months. But more recently, an abbreviated schedule of one and six months has been adopted in the EU, Canada, and more recently in the United States for individuals less than 15 years old, based upon the finding that those two doses in the younger age groups is equivalent in the antibody responses that are induced in comparison to the three doses in young adults, in which the efficacy was demonstrated in the clinical trials. So, this slide shows a timeline of HPV association with cervical cancer versus vaccine development. And the take-home message here is that basic development to a public health intervention generally takes decades. So, HPV16 was first discovered in 1982 by Harald zur Hausen and his colleagues in Germany, and he later went on to win the Nobel Prize for that discovery. It took us 10 years in the laboratory to develop an attractive HPV vaccine particle that we could show generated high titers of antibodies that could prevent infection. And that was in 1992. Now, during the decades of the ’90s and ’80s, there was a whole series of studies — case control and prospective epidemiological studies, and also laboratory studies, some of which my group was involved with — that led to the conclusion that HPV infection was the first necessary cause of a cancer, in this case, cervical cancer. And this correlated with the time in which we conducted the preclinical animal studies, the proof-of-concept studies, and started clinical trials, again in 1999, leading to the first efficacy data in 2001, licensure in females in 2006 and in males in 2009. And you can see that even though in 1992 we had the prototype for what was going to become the commercial vaccine, it still took 15 years in order for it to be widely distributed in people. Now, an important thing to note is that the basic understanding of how HPV causes infec… causes cancer was critical for developing these vaccines. And that’s because we wanted a vaccine that could be licensed for preventing cancer, but we could not use cancer as a primary endpoint in the trial. And there are two reasons for this. One, what we’re trying to do is prevent initial infection, and it takes decades to go from initial infection to cancer, which would be far too long for a controlled clinical trial. And secondly, because we have PAP screening, we identify premalignant lesions and have to remove them. And so it’s unethical in a trial with active follow-up to allow women to go on to cancer. But fortunately, by that time we understood that these premalignant lesions, particularly CIN3 — cervical intraepithelial neoplasia grade 3 — was a necessary precursor for cervical cancer. And the FDA and other regulatory bodies were convinced that if we could demonstrate protection against intermediate- and high-grade cervical lesions that we would be preventing future cancers, and therefore could license the vaccine for prevention of cancer. And so there were several large-scale trials that were done, then, to demonstrate that these vaccines could work against disease endpoints. And remarkably, the vaccines were extremely effective in these efficacy trials. This is the first vaccine against a sexually transmitted disease that has shown any substantial efficacy. And what you can see here is that for both Cervarix and Gardasil, if you look in women who had no genital HPV infection at entry and restrict the analysis to the types that are targeted by the vaccine, that there was 100% protection against the most cancer-proximal endpoint we could look at, which was CIN3. There was also strong protection against genital warts with Gardasil, which contains HPV6 and 11. Now, the protection in men looks as if it may be slightly less, but it’s important to note that we think that we miss more prevalent infection at the time of vaccination in men, and so that’s… a lot of these, what look like breakthrough infections, are actually emergence of prevalent infections that pre-exist prior to vaccination. Now, Gardasil-9, which again was approved in late 2017, the clinical trials were a little different. Because at that point there were two vaccines that were already FDA-approved. And so it was considered unethical to test them against the placebo control. So, what the FDA and other regulat… regulators agreed to is to demonstrate that they could protect against high-grade dysplasias against the new types in Gardasil-9, the five oncogenic types, and had immunogenicity equivalents to the four types that were already there. In other words, the antibody levels were no different to the four types in common. And what they found in this trial is that there was 96% efficacy against CIN2/3 of five new types in comparison, direct the comparison, to Gardasil-4, and the antibody types to the common VLPs… the antibody titers to the common VLPs were no different. So, in addition to having demonstrated protection against neoplastic disease against cervical lesions, there is also data that it protects against anal lesions and vulvar/vaginal lesions in clinical trials. But as you can see here, there’s no evidence that it protects against lesions on the penis. And the reason for this is that penile intraepithelial neoplasias are actually relatively uncommon in comparison to the ones listed above. And the trials were not particularly large. And secondly, we don’t know that it protects against infections… lesions at the oro-pharynx, and the reason for this is the premalignant lesions have not been yet identified. Now, I think we have to start think… stop thinking about the HPV vaccines as a new kid on the block. It’s still considered one of the newer vaccines, but it’s already been commercially available for 12 years and has been licensed in 82 countries, and over 270 million doses have been given globally. So, I think we have to consider this just one of the standard, routine vaccines to deliver to adolescents. Now, we think that the way this vaccine works primarily is that it protects from initial infection by generating antibodies that bind the virus and prevent them from initially infecting people. And the reason we think that is that most vaccinees never test positive for HPV DNA using sensitive PCR assays. And also the fact is that when we do see breakthrough infections, they happen predominantly early during the trials. So, why would we get more infections early rather than later, when normally immunity is waning? And the reason, again, we think this is because what we’re actually measuring is emergence of infections that were present at the time of vaccination, who then emerged during the vaccination period. And so really it could not be prevented by a prophylactic vaccine. So, it’s also important, although these vaccines seem like they’re very potent at preventing initial infections by the types targeted by the vaccine, there are some things that the vaccines don’t do. And they don’t prevent infection or disease by most of the other HPV types. There’s a few very closely related types that we do see partial protection from it. And lastly, it’s important to note that they do not induce clearance of infection or prevent progression of lesions once they’ve occurred. And this is why it’s really very important, to get the biggest bang for the buck, to vaccinate young women and young men before they become sexually active, because once they have the infections the vaccine is not going to do any good for that particular infection. Now, it’s important to note that despite what maybe you can read if you google HPV vaccines and safety, the vaccines actually have an excellent safety record. So, low grade and transient infection site reactions are common. And when there are systemic reactions, such as fever, they’re generally mild and self-limiting. A certain number of the vaccinees do faint during the vaccination. But we believe that this is primarily due to seeing the needle, and the fainting is no more than other needle injections in this age group. Important to point out is that there’s no pattern of serious adverse events, either in the trials or in post-licensure surveillance, that would suggest that there’s a causal relationship of the vaccine to any serious adverse event. So, turning to effectiveness, in vaccinology we have two different terms. Efficacy means how well the vaccine works in a clinical trial. And effectiveness is used to describe how well the vaccine uses in general use, in the public, for instance in national immunization programs. And we’re starting to accrue, now that the vaccines have been available for more than a decade, substantial evidence that these vaccines are also very effective in general use. And there’s data both in the levels of infections, prevention of genital warts, and prevention of cervical lesions. And I’m just gonna give you a little flavor of some of the data that’s starting to come out from some of these national immunization programs. So, this slide shows the effect of Cervarix vaccination in England after the beginning of their vaccination program. And what it shows is that, in the younger age group, 16 to 18, in which 50% are vaccinated by year two or three, there was a 50% reduction in the prevalence of HPV16 and 18 in the population. And after three to four years… after four to six years, when over 70% were vaccinated, there was a 76% drop in the prevalence in the population. But it’s important to note that in the older young women, who… there was no vaccination, there was no drop in HPV infection rates. And as a matter of fact, it may have even been increasing slightly. There’s also evidence, for instance this evidence from Australia, that the rate of intermediate- and high-grade dysplasias is also dropping in the vaccinated population. So, here you see that in under-18-year-olds, the rate of CIN2/3 is declining rapidly since the start of vaccination, which is indicated by the needle; whereas we maybe are starting to see a little bit of a drop in the 18-to-20-year-olds, where there’s some vaccination but relatively poor coverage; and no drop in the older women, who are not being vaccinated. Now, this vaccine, then, therefore we think has an enormous potential for decreasing the burden of HPV-associated disease. And this shows what we think the potential is for cervical cancer. So, in a non-vaccinated scenario, the expectations are that, over the next 65 years, worldwide we will see 19 million cases of cervical cancer and 10 million deaths. And you can see that the majority of these cases will occur in lower/middle-income and lower-income countries. Now, in the ten years since we’ve initiated the vaccine programs worldwide, it’s been estimated that we’ve prevented a little over 350,000 cases and 150,000 deaths. And the majority of these are in upper-income countries or high-income countries. And again, the reason for this is that this is where the vaccine is being delivered. And so, if you look at this number, 150,000, from a preventive point of view, we’re disappointed. But this sort of points out the real potential of cancer prevention. Because if we had shown you that 150,000 women were cured of cancer, everybody would be really excited about it. And it points out that for prevention, the individual who’s not going to get the cancer doesn’t know it. And so it can tend to be underappreciated in relationship to cancer treatments. And it’s not only in low-resource countries that this vaccine is being underutilized, but also in countries like the United States. So, this slide shows the uptake over time since the introduction of two other vaccines for adolescents versus the HPV vaccine. And what you can see is that the three-dose coverage is only a little above 30%, and the one-dose coverage is about 50%, which is much less than the other two vaccines. So, what do… what can we do to increase uptake, particularly in low-resource settings? Well, importantly, both [companies] have committed to sell the vaccine to GAVI, which is a global alliance that buys vaccines and distributes it to the 72 lowest-income countries in the world. They decided to buy it for five dollars a dose, which is much, much less than the market price. Another possibility that we’re working with is with vaccine manufacturers in emerging countries, for instance in India and China and Brazil, who, again, could potentially manufacture the vaccine and sell it at a lower price. We certainly need to address vaccine hesitancy, particularly in families but also in health care providers who seem to be a major impediment to increasing uptake of the vaccine. And finally, that’s something I’ll deal with now, is that what happens if we can deliver fewer than three doses. It’s problematic trying to set up vaccination programs where adolescent girls, and in some cases boys, have to come in three times in six months. And so we want to determine whether we can deliver fewer than three doses, for instance even a single dose. And the reason we think this might be possible comes from post hoc analysis of trials, for instance this NCI sponsored trial in Costa Rica, which used Cervarix. And what this table shows is that if you look at the end of seven years, the total number of HPV16 and 18 infections in women who got… who got three doses, which is what they were supposed to get, versus the women who for various reasons only got one dose, is no different. The confidence intervals are wider for the one dose, because there’s actually relatively few women who didn’t get the full three doses, but they’re completely overlapped. If you look at the types in which we get partial protection, HPV31, 33, and 35, you can see that, again, there’s no difference in the total number of infections by these types at the end of seven years. And if you look at other oncogenic or nononcogenic types, which we know the vaccine does not protect against, again, the accumulated incidence is the same. And this is an important observation because what it says is that the women who get only one dose are not some unusual subset that have lower exposure rates to genital HPV infections. This is just another way of looking at the data, where if we just look cross-sectionally at the end of seven years… who has infections? So, in comparison to the control group that didn’t get vaccinated, you can see that there’s essentially no difference between the women who got one, two, and three doses for nononcogenic types, for the cross-reactive types shown in red, or for the targeted types, HPV16 and 18, shown in blue. So, there are actually two other trials, again, post hoc analysis, that demonstrated that at the end of four years, for instance in the GlaxoSmithKline, the corporate-sponsored trial, they also showed similar efficacy at one, two, and three doses. And for Gardasil, there was an interrupted Indian cluster randomization trial, where they were supposed to compare two doses to three doses, but for political reasons the vaccine trial was stopped, and so some of the women only got one dose. And again, after seven years, they showed similar protection in women regardless of the number of doses they received. So, the question then is, well, now, is it time? Should we adopt a single dose vaccination program based upon this data? Well, we believe that because these are this post… these… these findings are post hoc, that they provide insufficient evidence to generally promote implementation of single dose HPV vaccination programs. Now, they may possibly induce early adoption, and particularly in low-resource settings where they can’t afford to give three doses or even two doses, with the contingency that if one dose doesn’t appear to work that the women can then be boosted. That type of implementation issue or program may be justified. But we think that to really put this all on very firm, strong footing, and be able to convince regulators to approve single dose vaccination programs, we need to do a well-controlled randomized controlled clinical trial to compare one and two doses. And the NCI has now undertaken this study in Costa Rica with support from the Gates Foundation. And it’ll be a four-armed trial where we’ll be comparing one and two doses of Cervarix, and one and two doses of Gardasil-9, with 5,000 12-16-year-old young ladies in each arm. We’ll do a survey of HPV prevalence in unvaccinated young women in the same region of the same age so we can get some sort of idea of pseudoefficacy. And we expect the trial to last for four years with long term follow up. So, to conclude, the HPV vaccines are safe and very effective at preventing incident infection and disease by the vaccine-targeted types. The vaccines have great potential for reducing the burden of HPV-induced cervical and other types of cancers worldwide. So the primary challenge now is implementation, to see that the vaccines reach the individuals who are most in need in them. And we believe that the way to maximize implementation is to demonstrate that a single dose is enough in a randomized clinical trial, which could be transformative for implementation of this vaccine. So, I would like to conclude by thanking some of the collaborators that have been working in my lab and in other places around the world. And particularly, Doug Lowy, who’s been my co-PI on all the work that happened in our lab related to the HPV vaccines for the last 20 years. So, thank you very much.

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