HPV vaccines—trying to fill the Pap gap

October 2005
PAP/NGC Programs Review

William D. Tench, MD

Prophylactic, and to some extent therapeutic, vaccines against human papillomavirus—the caus a tive agent of cervical neoplasia—have garnered much interest and momentum as a means of cervical cancer control.

The Papanicolaou test is the most successful cancer screening tool in history. In countries where it has been widely implemented, deaths from cervical cancer have declined significantly. However, even in countries with a substantial screening infrastructure, cancer deaths continue to occur because a significant group of patients remain unscreened for a variety of reasons, including regional lack of ready access, lack of education, and personal resistance. Hence in the United States, approximately 15,000 incident cases of invasive cervical cancer occur each year with about 4,000 deaths—despite an expenditure of about $6 billion to detect and eradicate cervical abnormalities, most of which are low grade and self-limited.¹

On a worldwide scale, the problem is much more severe. The majority of the world’s population does not have access to any screening program, and as such, as many as 400,000 incident cases of cervical cancer occur worldwide, with as many as 200,000 deaths.²After lung and breast cancer, cervical cancer is the No. 3 killer of women and in many regions is No. 1. Some studies have found that cervical cancer may show more reduction in life expectancy than do other commonly perceived mortal diseases such as AIDS and tuberculosis.³ Implementing widespread cancer screening in countries that now have limited or no resources to devote to it would be nearly impossible, other than on a narrow or focused scale taking in only a small percentage of the overall population.

Therefore, to diminish this significant cause of mortality, other methods of prevention or therapy are required.

To understand HPV vaccine development, it is important to have a basic knowledge of HPV biology. It is well known that essentially all cervical cancers (99.7 percent documented⁴) are associated with HPV. This association is stronger than that noted for smoking and lung cancer and on the order of the association of hepatitis B and hepatocellular carcinoma.⁵ There are more than 100 distinct HPV types, of which 20 to 30 are known to infect the genital tract. Of these, the most commonly found are types 16 and 18, which together account for about 70 percent of cancers worldwide. With the addition of a further group of five (45, 31, 33, 52, 58), nearly 90 percent of all cervical cancers are included with little regional variation noted.⁶ After these seven types, some regional differences in HPV type association with cancer are noted. HPV types causing cervical cancer are referred to collectively as “high risk.” In addition, other HPV types (most notably types 6 and 11) are collectively referred to as “low risk” because they are not associated with the development of cancer but are instead associated with benign but nonetheless clinically significant disease, most notably genital tract papillomas or condyloma accuminatum.

The molecular biology of HPV and cervical carcinogenesis has been well elucidated during the past 15 years. The HPV genome is made up of an approximately 8 kbase double-stranded DNA arranged in a circular configuration. It is made up of seven early (E) genes, which control DNA replication and transcription, and two late (L) genes, which code for the proteins that form the intact viral capsid structure. In the vast majority of cases, the viral infection/transmission cycle is a benign event. Direct skin-to-skin contact, predominantly but not exclusively via sexual relations, allows HPV virions to infect the metabolically active squamous metaplastic cells of the genital tract. Intact viral genome is transcribed, and intact virions are formed that can then go on to infect other cells (and individuals). It is during this process that cytologists note the classic koilocytotic viral cytopathic effect known as low-grade squamous intraepithelial lesion, or LSIL. These infections are predominantly transient and benign, and they clear spontaneously in greater than 90 percent of cases when followed for a two- to three-year period. The true neoplastic sequence occurs in the population of patients that do not clear their infections (persistent infections).⁷

The development of high-grade dysplasias (true neoplastic precursor lesions) is associated with specific events that include the integration of portions of the HPV genome into the host genome. The early genes E6 and E7 are always conserved in such events. In addition, breaking of the circular viral DNA nearly always occurs in the region of E2, which is responsible for regulatory control of transcription of E6 and E7, thus leading to loss of this function. E6 transcription product is known to bind to and inhibit the tumor suppressor activity of p53, allowing the cell to enter DNA replication (S phase of the cell cycle) without the benefit of normal DNA repair functions (loss of normal G1 arrest). In a like fashion, E7 protein product binds to the retinoblastoma tumor suppressor gene product (pRb), and this interaction, via a path of downstream effects, allows cells to proceed uninhibited through the S-phase. Overall, the effects caused by loss of regulation of these two early genes lead to uncontrolled DNA synthesis and cell proliferation with host genes not having the benefit of normal presynthesis repair, leading to accumulations of cells having mutated DNA. Because this neoplastic process does not involve the entire viral genome, intact virions are not produced, and the neoplastic cells show no evidence of the koilocytotic viral cytopathic effect noted in LSIL.⁸

Vaccine development is broadly divided along two conceptual lines. Prophylactic vaccines are those that seek to eliminate primary infections by HPV, while therapeutic vaccines are those that will provide benefit against present infections, HPV-associated neoplastic processes, or both.

Prophylactic vaccines in development now target viral capsid epitopes, most commonly L1, the major capsid protein. For vaccine delivery, the antigens are packaged in artificially synthesized structures called virus-like particles (VLP), the L1 components of which are manufactured in bacterial or other cellular expression systems, self-assemble, and can then be introduced into patients, currently by intramuscular injection. VLPs are highly immunogenic and elicit potent neutralizing antibodies and some cellular immune responses. HPV has exquisite type specificity, however, and L1 VLP-based vaccines require having L1 epitopes from each HPV type desired, as essentially no cross-reactivity responses between the types are noted.

Phase I clinical trials of prophylac tic vaccines developed against a number of HPV types have shown that the vaccines are safe and well tolerated.^9,10 In phase II studies, Koutsky, et al showed 100 percent efficacy of an HPV 16 L1 VLP vaccine in preventing persistent HPV infections and cytologic abnormalities in a randomized two-arm study comparing vaccine and placebo. During the trial, 41 cases of persistent HPV 16 infection and nine cases of cervical intraepithelial neoplasia, or CIN, were identified in the placebo arm, with no such cases identified in the vaccine arm (P<.001). Of interest was that incident HPV 16 infections were identified in the vaccine arm, but these never became persistent, the marker used as one endpoint for vaccine efficacy (CIN being the other).¹¹ In another phase II trial of a bivalent HPV 16/18 L1 VLP vaccine, Harper, et al showed 100 percent efficacy against persistent HPV 16 and 18 infections and 93 percent efficacy against HPV-16/18-associated cytologic abnormalities, all of which were less than HSIL.¹² In yet another phase II study of a quadrivalent vaccine targeting HPV 16/18/6/11, a reduction of persistent infection by these viral types of 90 percent was achieved.¹³

Phase III studies are reportedly underway now. Merck & Co. is testing a quadrivalent vaccine targeting HPV 16/18/6/11 (trade name Gardasil) with the intent of allowing protection against genital condyloma in addition to cervical carcinoma. This trial is reported to be entering more than 20,000 subjects at over 100 clinical sites worldwide. GlaxoSmith Kline is also in phase III trials with a vaccine against HPV 16/18 (trade name Cervarix) and is enrolling more than 30,000 subjects. Results from these studies as well as Food and Drug Administration applications are expected in 2006.¹⁴ Future vaccines might be expected to contain additional epitopes against other prevalent HPV types. As noted by Munoz, et al, a vaccine containing a cocktail of the seven most prevalent high risk types would provide 90 percent coverage of all high-risk infections with little regional variability.⁶

What is the anticipated effect of introducing vaccines into the population? Mathematical models have been developed that explore the implications of implementing prophylactic vaccine in various settings. Goldie, et al predicted that an HPV 16/18 specific vaccine that prevented 98 percent of persistent infections would reduce cervical cancer in a population by 51 percent. However, in their model, reductions in incident LSIL would be negligible in the same population because of the increase in infections caused by other non-covered HPV types stepping in to “fill the void” left by the eradication of type 16/18 infections.¹⁵ In another modeling study, Taira, et al showed that even modest penetrance of a 16/18 specific vaccine into the population (as low as 50 percent) would give nearly a full effect of about 62 percent reduction of cervical cancer.¹⁶ Adapting the same model to heavily screened populations, Goldie, et al also showed that the most cost-effective overall management strategy would be vaccination at age 12 years with triennial screening (with conventional cytology) beginning at age 25 years.¹ Such models have significant implications for the overall planning of continued screening programs. Clearly, screening must continue; however, to realize the savings expected with vaccine introduction, programs may look different in structure.

A number of unanswered questions about prophylactic vaccines remain. The first is when to vaccinate. Investigators indicate that vaccination before onset of sexual activity (10 to 15 years) makes the most sense, but targeting this age group may bring opposition from parental groups who may contend that vaccination for a sexually transmitted disease in this age group could increase subsequent promiscuity.¹⁷ However, data do suggest that parents fully informed about HPV, cancer development, and vaccines would favor having their children (and themselves) vaccinated.¹⁸ In addition, data from the Merck trials have shown that vaccinating younger individuals (10 to 15 years) caused a greater immune response than did vaccinating older individuals (16 to 23 years), indicating that the effect may be more pronounced on less mature immune systems.¹⁷

The second unanswered question is how long the immune protection will last and whether boosters will have to be given. Studies to date show that protective immunity lasts for several years, but results beyond this time frame are not yet known. The aforementioned mathematical models use 10-year intervals for boosters.

Third, should adolescent males be vaccinated in addition to females? In the Taira, et al model, male vaccination suggested only a small increment of cervical cancer decline (2.2 percent) over female vaccination alone.¹⁶ However, the so-called effect of herd immunity is not well studied in this population, and suggestions have been made that increasing the overall rate of vaccination among males and females, and hence reducing infections in both sexes, may have the effect of more substantial reductions of neoplastic disease in women.¹⁹

Fourth, what should be the mode of inoculation? Parenteral vaccination may be easily adopted in countries with well-established vaccination programs, but in developing countries this method of administration may not be widely applicable. Interestingly, attempts to genetically engineer foods to contain antigenic L1 components have shown success in eliciting immune responses in animals.²⁰ These results may suggest a potential simplified and more widely available delivery system in the future, but many questions about other effects of such engineered foods on the population will have to be addressed.

Therapeutic vaccines that will affect prevalent HPV infection and associated neoplasia are much less developed than are the abovementioned prophylactic vaccines. Based on the HPV biology earlier noted, such vaccines would need to use epitopes retained in transformed cells, most likely E6 and E7, in distinction to the capsid antigens found in the infective stages. To have effects against already transformed host epithelial cells, therapeutic vaccines will require cellular immune responses to be elicited. A number of studies have been published using E6 and E7 epitopes with variable success. While the specifics of antigen delivery are beyond the scope of this article, methods used to date have included viral vectors such as Vaccinia, small peptides, proteins, bare or encapsulated DNA, and direct inoculation of antigen into autologous dendritic cells. For further reading on methods of antigen presentation, the reader is referred to the excellent review article by Schreckenberger and Kaufmann.²¹ Although results to date using therapeutic vaccines have shown promise, further development and study will be required with all such modalities.

In summary, prophylactic vaccine development is now in a mature stage and commercial products are expected within the next several years. Implementation could significantly reduce cervical cancer mortality in screened and unscreened populations. In addition, use of the vaccine in developed nations will probably initiate a paradigm shift in current screening protocols. Screening will most likely occur less often, start at a later age, and may be by different methods than are being employed today. Therapeutic vaccines are at an earlier stage of development, and it is too early to predict what forms such “immunotherapy” will take and what their overall effects might be. For both vaccine approaches, progress is happening rapidly, and this bodes well for the population affected by this preventable disease.

Resources

1. Goldie SJ, Kohli M, Grima D, et al. Projected clinical benefits and cost-effectiveness of a human papillomavirus 16/18 vaccine. J Natl Cancer Inst. 2004; 96: 604–615.

2. Durst M, Gissmann L, Ikenberg H, zur Hausen H. A papillomavirus DNA from cervical carcinoma and its prevalence in cancer biopsy samples from different geographic regions. Proc Natl Acad Sci USA. 1983;80:3812–3815.

3. Lehtinen M, Dillner J, Knekt P, et al. Serological diagnosis of human papillomavirus type 16 infection and the risk for subsequent development of cervical carcinoma. BMJ. 1996;312:537–539.

4. Walboomers JM, Jacobs MV, Manos MM, et al. Human papillomavirus is a necessary cause of invasive cervical cancer worldwide. J Pathol. 1999;189:12–19.

5. Rohan TE, Burk RD, Franco EL. Toward a reduction of the global burden of cervical cancer. Am J Obstet Gynecol. 2003;189:S37–S39.

6. Munoz N, Bosch FX, Castellsague X, et al. Against which human papillomavirus types shall we vaccinate and screen? The international perspective. Int J Cancer. 2004;111:278–285.

7. Schiffman M, Castle PE. Human papillomavirus. Epidemiology and public health. Arch Pathol Lab Med. 2003;127:930–934.

8. Jung W-W, Chun T, Sul D, et al. Strategies against human papillomavirus infection and cervical cancer. J Microbiol. 2004; 42:255–266.

9. Evans TG, Bonnez W, Rose RC, et al. A phase 1 study of a recombinant viruslike particle vaccine against human papillomavirus type 11 in healthy adult volunteers. J Infect Dis. 2001;183:1485–1493.

10. Harro CD, Pang YY, Roden RB, et al. Safety and immunogenicity trial in adult volunteers for a human papillomavirus 16 L1 virus-like particle vaccine. J Natl Cancer Inst. 2001;93:284–292.

11. Koutsky LA, Ault KA, Wheeler CM, et al. A controlled trial of a human papillomavirus type 16 vaccine. N Engl J Med. 2002;347:1645–1651.

12. Harper DM, Franco E, Wheeler C, et al. Efficacy of a bivalent L1 virus-like particle vaccine in prevention of infection with human papillomavirus types 16 and 18 in young women: a randomised controlled trial. Lancet. 2004;364:1757–1765.

13. Villa LL, Costa RL, Petta CA, et al. Prophylactic quadrivalent human papillomavirus (types 6, 11, 16, and 18) L1 virus-like particle vaccine in young women: a randomised double-blind placebo-controlled multicentre phase II efficacy trial. Lancet Oncol. 2005;6:271–278.

14. Washam C. Two HPV vaccines yielding similar success. J Natl Cancer Inst. 2005;97:1030.

15. Goldie SJ, Grima D, Kohli M, Wright TC, Weinstein M, Franco E. A comprehensive natural history model of HPV infection and cervical cancer to estimate the clinical impact of a prophylactic HPV-16/18 vaccine. Int J Cancer. 2003;106:896–904.

16. Taira AV, Neukermans CP, Sanders GD. Evaluating human papillomavirus vaccination programs. Emerg Infect Dis. 2004;10:1915–1923.

17. Washam C. Targeting teens and adolescents for HPV vaccine could draw fire. J Natl Cancer Inst. 2005;97:1030–1031.

18. Davis K, Dickman ED, Ferris D, Dias JK. Human papillomavirus vaccine acceptability among parents of 10- to 15-year old adolescents. J Low Genit Tract Dis. 2004;8:188–194.

19. Geipert N. Vaccinating men for HPV: new strategy for preventing cervical cancer in women? J Natl Cancer Inst. 2005;97: 630–631.

20. Sasagawa T, Tani M, Basha W, et al. A human papillomavirus type 16 vaccine by oral delivery of L1 protein. Virus Res. 2005;110:81–90.

21. Schreckenberger C, Kaufmann AM. Vaccination strategies for the treatment and prevention of cervical cancer. Curr Opin Oncol. 2004;16:485–491.

Dr. Wilbur, vice chair of the CAP Cytopathology Committee, is director of cytopathology at Massachusetts General Hospital, Harvard Medical School, Boston.