Identification of a viral agent such as HPV as a cause of disease(s) implies that successful prophylactic or therapeutic intervention against the viral agent should prevent the disease(s) it causes. A preexisting viral infection can theoretically be targeted by an antiviral or a therapeutic vaccine. Successful antivirals have been developed for the treatment of some viral infections, including diseases such as HIV and influenza (19
), but not against viruses such as HPV. A therapeutic vaccine against HPV infection would be highly desirable to prevent the cancer-associated complications of HPV infection, which only develop after many years of infection. However, despite ongoing efforts to develop effective therapeutic vaccines against HPV and other viral infections, none has been shown to be highly effective clinically (20
), probably because the vaccines have not yet adequately mimicked critical aspects of a curative immune response. On the other hand, prophylactic vaccines have been developed against a variety of human viral pathogens and are often a cost-effective approach to interfere with the diseases caused by these pathogens (21
). To be widely implemented, a prophylactic vaccine generally needs to confer high-level protection for at least several years without boosting and to be particularly safe, as it is given to healthy individuals.
The recognition of HPV as the cause of cervical cancer and other diseases therefore implied that an effective HPV vaccine should be able to interfere with the benign and malignant conditions attributable to HPV infection. However, approved prophylactic vaccines have been directed against infectious agents that cause systemic disease, and efforts to develop vaccines against sexually transmitted agents such as HPV whose disease results from local infection had not proven successful. It is believed that neutralizing antibodies form the cornerstone of most prophylactic vaccines, and viruses that cause disease only after passing through the circulation are accessible to the neutralizing antibodies present in the blood (22
). Another limitation was that the presence of oncogenes in HPVs suggested that a subunit vaccine approach would theoretically be preferable to an inactivated vaccine or an attenuated live-virus vaccine, and it was unclear whether a subunit HPV vaccine would have the potential to be effective against the local infections caused by genital HPVs.
Despite these uncertainties, 2 pharmaceutical companies, Merck and GlaxoSmithKline, have recently reported a remarkable degree of protection by candidate prophylactic HPV vaccines (see below). The vaccines that both companies are developing are subunit virus-like particle (VLP) vaccines composed of a single viral protein, L1, which is the major structural (capsid) protein of the virus and contains the immunodominant neutralization epitopes of the virus. The vaccines are based primarily on preclinical research showing that (a) when expressed in cells, L1 has the intrinsic ability to self-assemble into VLPs (Figure ) (23
) that can induce high levels of neutralizing antibodies (23
); (b) in animal models of animal papillomavirus infection, parenteral vaccination with L1 VLPs protects from high-dose challenge with homologous virus (29
), while animals are not protected by systemic immunization with denatured L1 or L1 VLPs from a heterologous papillomavirus because L1 neutralization epitopes are conformationally dependent and predominantly type specific (29
); and (c) protection can be passively transferred by immune IgG (29
). The VLPs from the Merck vaccine are produced in yeast (32
), while the VLPs from the GlaxoSmithKline vaccine are produced in insect cells via recombinant baculovirus (35
). Merck uses alum as an adjuvant in its vaccine, while GlaxoSmithKline uses AS04, a proprietary adjuvant composed of alum plus monophosphoryl lipid A (a detoxified form of lipopolysaccharide). Both vaccines use purified particles, which are given as 3 intramuscular injections over a 6-month period.
Electron micrograph of HPV16 L1 VLPs.
Of course, it would be desirable for an HPV vaccine to have the ability to prevent all cases of cervical cancer. However, although the 15 oncogenic types implicated in cervical cancer are more closely related to each other phylogenetically than they are to the HPVs that cause nongenital skin lesions (warts) (36
), the immunodominant epitopes in L1 VLPs induce neutralizing antibodies that are predominantly type specific (37
). It has therefore been necessary, at least for the first generation HPV vaccines, to focus on the HPV types found most frequently in cervical cancer. This consideration has led both companies to focus on HPV16 and HPV18, which, as noted above, account for about 70% of cases of cervical cancer. The Merck vaccine also targets HPV6 and HPV11, which together account for about 90% of external genital warts (38
); the latter 2 types also infect the cervix, but are not implicated in cervical cancer. Thus, the GlaxoSmithKline vaccine that is currently in phase III trials is a bivalent vaccine composed of VLPs from HPV16 and HPV18, while the vaccine that Merck has used for its phase III trials is a quadrivalent vaccine that contains VLPs from HPV6, HPV11, HPV16, and HPV18.
When considering appropriate end points for determining vaccine efficacy, the most relevant end points recommended by an FDA vaccine advisory panel (39
) were a reduction in the incidence of vaccine type–specific persistent infections and of associated moderate- and high-grade cervical dysplasias and carcinomas in situ, which together are referred to as cervical intraepithelial neoplasia 2+ (CIN2+; CIN is graded as CIN1, CIN2, and CIN3 for low-, moderate-, and high-grade dysplasia, respectively). HPV DNA testing results have been shown to be substantially more reproducible than the pathological diagnosis of dysplasia (40
), but moderate- and high-grade dysplasias represent clinical end points that trigger therapeutic intervention. It is important to note that it would be unethical to use cervical cancer as a primary end point for vaccine efficacy trials, as cervical cancer screening can prevent the vast majority of cancer through the identification of precancers, which are then treated. Also, the interval between infection and the development of invasive cancer usually takes more than 10 years (7
Following the observations that systemic vaccination with a monovalent HPV16 L1 VLP vaccine was safe and highly immunogenic (41
), even without adjuvant, a Merck-sponsored proof-of-principle efficacy trial of an HPV16 L1 VLP vaccine reported that fully vaccinated women who were HPV negative throughout the vaccination period were completely protected against the development of persistent incident infection with HPV16 when followed for an average of 17 months (Table and ref. 32
). This vaccine cohort has now been followed for an average of 3.5 years, and the high level of protection was maintained throughout this period (Table and ref. 34
). After the initial peak, the levels of serum antibodies appeared to decline approximately 10-fold over the first 2 years following vaccination but then remained stable for the remainder of the follow-up period. If the level of serum antibodies represents a surrogate for protection against infection, the stability of the antibody titers would suggest that high-level protection may continue substantially beyond 3.5 years. Preliminary efficacy results from Merck’s quadrivalent vaccine were also published and showed excellent protection against the viruses targeted by the vaccine (Table and ref. 33
). In unpublished studies, Merck has reported at scientific meetings on the clinical efficacy of the multinational phase III trial of their quadrivalent vaccine, with an average follow-up of 1.5 years (42
). When fully vaccinated women who remained negative for infection throughout the vaccination period were analyzed against the comparable placebo cohort, the vaccine was 100% effective in preventing CIN2+ associated with HPV16 or HPV18 and also in preventing external genital warts associated with HPV6 or HPV11. Even when the efficacy was compared starting 1 month after the first immunization, protection for these end points was greater than 90%. Thus systemic immunization with a subunit HPV vaccine can achieve a high degree of protection in women against benign and premalignant diseases induced by this sexually transmitted local infection of the genital mucosa or skin.
Proof-of-principle HPV VLP prophylactic efficacy trials
The Merck-sponsored trials have not reported on the comparative incidence of infection by other HPV types not included in the vaccines. However, the proof-of-principle monovalent HPV16 trial has reported on the total number of patients with various grades of cervical dysplasia, and whether the dysplasias were associated with HPV16, for the fully vaccinated women who were HPV negative throughout the vaccination period (32
). In contrast to the complete protection seen against HPV16-associated dysplasias, there was no difference between the placebo and vaccinated groups in the number of non–HPV16-associated dysplasias in the initial report on this cohort. In the 3.5-year follow-up report (34
), there was also no evidence that non–HPV16-associated dysplasias in the vaccinated group were less frequent in number than those in the nonvaccinated group. These results strongly suggest that the protection conferred by Merck HPV16 monovalent vaccine was predominantly HPV type specific.
GlaxoSmithKline has also sponsored a proof-of-principle efficacy trial of an HPV16 and HPV18 bivalent vaccine similar to the one that is currently in phase III trials (Table and ref. 35
). Their placebo-controlled proof-of-principle trial had a smaller number of participants who were followed for an average of 18 months. In the group of women who were fully vaccinated and remained uninfected throughout the vaccination period, all subsequent persistent infections associated with HPV16 and HPV18 occurred in the placebo group, although there were only a total of 7 cases (5 with HPV16 and 2 with HPV18). When incident persistent infection was monitored starting 1 month after the first dose of vaccine, combined protection against HPV16 and HPV18 was still about 90%. Another potentially important result, which has been presented at meetings but not yet published, is that the vaccine has been associated with some cross-protection against HPV types closely related to HPV16 and HPV18, although this protection was less complete than that offered against HPV16 and HPV18 (44
). As Merck has not reported results analyzed in this manner, it is difficult to know whether the cross-protection represents an activity that may be greater in the GlaxoSmithKline vaccine. It will be important for the ongoing large-scale efficacy trials to analyze this parameter, as cross-protection against HPV types not in the vaccine could enhance its overall utility.