“Prime and pull” vaccination strategy for treatment of cervical pre-cancer: not quite ready for prime time

Surgical excision is the gold standard for treatment of cervical cancer precursors, referred to as high-grade squamous intraepithelial lesions (HSIL) or cervical intraepithelial neoplasia grades 2 and 3 (CIN 2/3). However, such excisional procedures have been associated with a risk of subsequent adverse pregnancy outcomes, including preterm labor and premature rupture of membranes (1). In a recent issue of Clinical Cancer Research, Sheth et al. present results from a randomized phase II trial of imiquimod with or without human papillomavirus (HPV) vaccination versus surveillance for treatment of CIN 2/3 (2). The primary objective of the trial was efficacy, defined as histologic regression to CIN 1 or less, of imiquimod with or without HPV vaccination compared to surveillance (control group). Secondary objectives included HPV clearance and treatment tolerability. Cervical CD4/CD8 T-cell infiltration before, during, and after treatment was compared as an exploratory objective.

Imiquimod, a Toll-like receptor 7 (TLR7) agonist, is approved by the United States Food and Drug Administration (FDA) for the treatment of HPV-associated genital warts and has been proposed as an alternative therapy for CIN 2/3 (3). By modulating the innate immune response via cytokine induction, imiquimod is hypothesized to improve HPV clearance. Randomized controlled trials have evaluated the effects of imiquimod versus excision (4-6) or imiquimod versus placebo (7) for the treatment of CIN 2/3. On average, these studies have demonstrated CIN regression rates of 50–73% for lesions treated with imiquimod, compared to 92–95% with excision. Studies of the effect of imiquimod versus excision on HPV clearance have shown mixed results, with some studies showing lower clearance with imiquimod, and some showing no difference by treatment modality, with clearance rates of approximately 43–69% (3,6).

The HPV vaccine comprises L1-virus like particles (VLPs) which produce type-specific antibodies to prevent infection with HPV. Unfortunately, the prophylactic vaccine is not effective as a therapeutic vaccine. While the late proteins L1 and L2 are important for HPV infection of the host cell, they are not expressed in cells that have already begun the process of malignant transformation (i.e., CIN 2/3 or cancer) and thus the currently available HPV vaccine is not effective against precancerous lesions or cancer. An observational study has shown HPV vaccination around the time of treatment of precancerous lesions to be associated with a decrease in risk of subsequent CIN 2+ by 43–67% (8) and is hypothesized to be due to prevention of re-infection with the same or different type of HPV. Two small randomized trials showed a decrease in the risk of subsequent CIN when the vaccine was administered post-surgery, but these studies are limited by small sample size and inclusion of CIN 1. Results from a recently presented randomized placebo-controlled trial of HPV vaccination following surgical excision of CIN 2/3 among 809 participants showed no significant difference in CIN 2/3 or HPV status at either 6 or 24 months [CIN 2/3 at 24 months, relative risk (RR) 0.67, 95% confidence interval (CI): 0.40–1.11, P=0.11] (9).

The current study by Sheth et al. was built on the concept of a “prime and pull” vaccination strategy in which the tissue is “primed” by triggering a systemic T-cell response with HPV vaccination (10). Activated T-cells are then “pulled” into the cervical mucosa with application of chemokines in the form of imiquimod, ultimately establishing long-term tissue-resident memory T-cells and protective immunity.

This study enrolled 133 participants with untreated CIN 2/3 into three arms: imiquimod alone (Arm B), imiquimod plus 9-valent HPV vaccine (Arm C), and surveillance (control, Arm A). The trial did not meet its pre-specified statistical outcome for efficacy (one-sided alpha of 0.025), with regression to CIN 1 or less in 84% of those in the imiquimod plus HPV vaccination group (P=0.38) and 95% in the imiquimod-only group (P=0.04) compared to 79% in the control group. Further, HPV clearance was not significantly different at 47%, 63%, and 50% in the combination, imiquimod, and control groups, respectively. While the trial met its accrual goal, the study may be underpowered to detect a difference. The power calculation was based on the assumption that the spontaneous regression rate would be 28–39% based on previously published data, and the regression in the control arm of this study was double the estimate at 79%. The spontaneous regression may have been higher than expected due to the younger age of the patients in this trial (median age 28 years) and the high prevalence of CIN 2 which is known to have a higher regression rate than CIN 3. While not statistically significant in this small sample size, 81.6% of participants in the control and imiquimod groups had CIN 2 compared to 68.4% in the combination group. Additionally, 71% of the participants in the control arm of this study had previously been vaccinated against HPV, compared to 63% in Arm B and 58% in Arm C. Of note, previously vaccinated individuals in Arm C received one dose of the 9-valent HPV vaccine during the study, while those previously unvaccinated received 2 doses. Studies of HPV type in CIN lesions in previously vaccinated populations show a type shift to HPV types not covered by the vaccine and which have a lower risk of progression and higher likelihood of spontaneous regression (11,12). Increased regression of CIN in previously vaccinated individuals is also suggested by methylation studies. DNA methylation of host-cell genes is associated with the regression of CIN. A study of a previously vaccinated cohort with a diagnosis of CIN showed low levels of DNA methylation compared to a mean reference methylation level among unvaccinated individuals, suggesting a higher likelihood of CIN regression among vaccinated individuals (13). The sample size in the current study is too small to perform meaningful subgroup analysis of regression by prior HPV vaccination status.

Data regarding the therapeutic efficacy of HPV vaccination in conjunction with imiquimod are limited, and no prior prospective studies have been published evaluating this combined therapy for treatment of CIN 2/3. However, a phase II trial of HPV vaccination plus imiquimod in patients with high-grade vulvar squamous intraepithelial lesions showed a regression rate of 63% (14). Evaluation of local immune response demonstrated increased local infiltration of CD4⁺ and CD8⁺ T cells in patients whose lesions showed a clinical response to treatment. Interestingly, Sheth et al. found that the average CD8⁺ T cell count was increased by 5-fold in the imiquimod group compared to the control group at mid-treatment (P<0.01), and, while not statistically significant, this group also demonstrated the highest CIN regression and HPV clearance; CD8⁺ T cell infiltration was not significantly different in the combination group compared to the control group. In a mouse model inoculated with TC-1 cells (a well-established cancer cell line expressing the HPV E7 oncogene and used as a model for human cervical carcinoma), Chuang et al. demonstrated that combined treatment with imiquimod and an HPV E7 antigen DNA vaccine enhances the E7-specific CD8⁺ T cell immune response (15). These results support the “prime and pull” vaccination strategy explored in Sheth et al. and suggest that perhaps a different vaccine target may better prime the tissue that is already undergoing malignant transformation.

Sheth et al. explore an interesting vaccine strategy, and the negative results reported in this trial should not lead to the abandonment of this concept altogether. The therapy was well-tolerated, and while we do not have data on long-term outcomes for imiquimod therapy for CIN with or without HPV vaccination, non-excisional therapy options for treatment of CIN are needed, both to decrease the risk of future pregnancy complications, as well as to decrease long-term effects on cervical architecture which can make subsequent evaluation of the cervix more challenging. Additionally, excisional treatment addresses the current CIN lesion but has relatively minimal effect on the underlying HPV infection, and treated individuals remain at risk for recurrent disease. Host immunologic response plays an important role in the clearance of HPV infection, and therefore may be a high-yield target for ongoing research regarding treatment of HPV-associated cervical intraepithelial lesions and even cancer.

Acknowledgments

None.

Footnote

Provenance and Peer Review: This article was commissioned by the editorial office, Translational Cancer Research. The article has undergone external peer review.

Peer Review File: Available at https://tcr.amegroups.com/article/view/10.21037/tcr-2024-2615/prf

Funding: None.

Conflicts of Interest: Both authors have completed the ICMJE uniform disclosure form (available at https://tcr.amegroups.com/article/view/10.21037/tcr-2024-2615/coif). D.G.K.T. has research funding from the US Department of Defense, Jounce Therapeutics, Moderna, and Glaxo Smith Kline. The other author has no conflicts of interest to declare.

Ethical Statement: The authors are accountable for all aspects of the work in ensuring that questions related to the accuracy or integrity of any part of the work are appropriately investigated and resolved.

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