A guide to STI prophylaxis

December 08, 2023

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In the United States, there were approximately 1.6 million cases of chlamydia, 710,000 cases of gonorrhea and 176,000 cases of syphilis reported in 2021, according to the CDC.

Anyone who engages in intercourse can get a sexually transmitted infection, but certain groups tend to be disproportionately affected, including people aged 15 to 24 years, men who have sex with men (MSM), and non-Hispanic Black people.

If left untreated, STIs can cause severe discomfort, increase the risk for more serious diseases such as HIV and potentially affect fertility.

Researchers are trying to identify new ways to prevent these infections. Below is a breakdown of the most recent study findings.

Doxy-PEP

As mentioned earlier, Chlamydia trachomatis is responsible for the majority of reported STIs in the U.S., and trachoma is the leading cause of infection-related blindness worldwide. Generally, the infection is asymptomatic, leading to increased unknown spread. For symptomatic patients, signs include vaginal discharge, bleeding, abdominal pain and dysuria.

First-line therapy for treatment is a 7-day course of doxycycline at 100 mg twice daily. To minimize transmission, patients should abstain from intercourse for 7 days and until any symptoms resolve and encourage partners to get treated to avoid reinfection, according to CDC guidelines.

In terms of preventive treatment, doxycycline postexposure prophylaxis (doxy-PEP) has shown promising results in a randomized open-label trial conducted in Seattle and San Francisco. The patient population being evaluated includes more than 500 MSM and transgender women living with HIV or taking HIV PrEP who had chlamydia, gonorrhea or early syphilis in the past year. The preliminary findings indicate that doxycycline taken at 200 mg within 24 to 72 hours of condomless sex reduced C. trachomatis infection compared with standard care, with RRs of 0.12 (95% CI 0.05-0.25) in the PrEP cohort and 0.26 (95% CI 0.12-0.57) in the HIV group. Studies among heterosexual and cisgender women are ongoing.

Syphilis is mainly transmitted via vaginal, anogenital and urogenital contact, but it can also be transmitted vertically, resulting in congenital syphilis, which has skyrocketed in the U.S. over the past decade.

Syphilis treatment varies. A single dose of benzathine penicillin G at 2.4 million units given intramuscularly works for primary, secondary and early latent syphilis in adults. For late latent syphilis, the treatment is 2.4 million units delivered intramuscularly in three doses at 1-week intervals. Neuro, ocular or otosyphilis among adults is treated with aqueous IV penicillin administered at 18 to 24 million units daily over 10 to 14 days.

In the doxy-PEP trial, there was a significant decrease in syphilis in the treatment group in the PrEP (RR = 0.13; 95% CI, 0.03-0.59) and HIV (RR = 0.23; 95% CI, 0.04 to 1.29) cohorts.

Gonorrhea, if left untreated, can lead to infertility, an increased risk for ectopic pregnancy and an additional risk for vertical transmission. Unfortunately, increasing rates of antimicrobial resistance have reduced the number of available treatment options. For uncomplicated infections in adults, the CDC has recommended a single dose of ceftriaxone 500 mg intramuscularly since 2020.

The doxy-PEP trial also revealed the potential utility of doxycycline for gonorrhea prevention. There were fewer gonorrhea infections in the PrEP (RR = 0.45; 95% CI, 0.32-0.65) and HIV cohorts (RR = 0.43; 95% CI, 0.26-0.71).

In October, the CDC issued draft guidance for the use of doxy-PEP among MSM and transgender women. It accepted public comments on the draft through Nov. 16. Although a French study called DOXYVAC also found that MSM were less likely to contract chlamydia, syphilis or gonorrhea if they took doxy-PEP, a third study conducted in Kenya did not see a significant reduction in infections among cisgender women for reasons that are unclear.

Meningococcal group B vaccine

The FDA approved the Neisseria meningitidis group B vaccine Bexsero in 2015. Recently, the vaccine has been studied as a potential vaccine against gonorrhea.

The first evidence of cross-reactivity of a meningococcal B vaccine to Neisseria gonorrhoeae came from New Zealand, where a large campaign in the early 2000s promoted meningococcal vaccination. Years later, researchers noted a decline in gonorrhea in the same time frame. Researchers studied the MeNZB vaccine — which is no longer available — retrospectively and found it had an adjusted vaccine effectiveness of 31% (95% CI, 21%-39%) against confirmed cases of gonorrhea among adolescents and adults aged 15 to 30 years, according to data published in The Lancet in 2017.

In DOXYVAC, an open-label randomized trial of STI prevention in MSM on PrEP, two study groups were randomly assigned to receive doxy-PEP within 72 hours of condomless sex or no PEP (2:1 ratio), and two shots of Bexsero or no vaccine (1:1). The primary endpoints that focused on the vaccine included incidence of first episode of gonorrhea 1 month after the second injection.

The study included 502 patients in the analysis. The incidence of a first episode of N. gonorrhoeae was 20.5 and 41.3 per 100 patient-years in the doxy-PEP and no PEP arms, respectively (adjusted HR = 0.49; 95% CI, 0.32-0.76). The incidence of a first episode of N. gonorrhoeae was 9.8 and 19.7 per 100 person-years in the Bexsero and no vaccine arms, respectively (aHR = 0.49; 95% CI, 0.27-0.88).

Bexsero is being studied as a potential gonorrhea vaccine in an NIH-sponsored phase 2, randomized, observer-blind, placebo-controlled, multisite trial that will include high-risk men and women aged between 18 and 50 years of age.

As of October, the trial had enrolled more than 1,500 of the targeted 2,200 participants, its principal investigator told Healio | Infectious Disease News.

The primary objective of the trial is to demonstrate the efficacy of Bexsero in the prevention of urogenital and/or anorectal gonococcal infections, with additional outcomes looking at adverse event rates and severity. The experimental group will receive the vaccine as a 1 mL dose twice, 2 months apart.

References:

• Bexsero. Package insert. GlaxoSmithKline; 2015
• CDC. Efficacy of doxycycline as PEP to prevent bacterial STIs. https://www.cdc.gov/std/treatment/doxycycline-as-pep-toe.htm. Last reviewed Aug. 28, 2023. Accessed Nov. 29, 2023.
• CDC. Gonorrhea. https://www.cdc.gov/std/gonorrhea/default.htm. Last reviewed April 11, 2023. Accessed Nov. 29, 2023.
• CDC. Primary prevention methods: Postexposure prophylaxis for HIV and STIs. https://www.cdc.gov/std/treatment-guidelines/clinical-primary.htm#CautionsForDoxyPEP. Last reviewed Sept. 29, 2023. Accessed Nov. 29, 2023.
• CDC. Sexually transmitted diseases surveillance 2021. https://www.cdc.gov/std/statistics/2021/default.htm. Last reviewed April 11, 2023. Accessed Nov. 29, 2023.
• CDC. STI treatment guidelines. 2021. https://www.cdc.gov/std/treatment-guidelines/. Last reviewed June 13, 2023. Accessed Nov. 29, 2023.
• CDC. Syphilis. https://www.cdc.gov/std/syphilis/default.htm. Last reviewed April 11, 2023. Accessed Nov. 29, 2023.
• Luetkemeyer AF, et al. Abstract 13231. Presented at: International AIDS Conference; July 27-Aug. 2, 2022; Montreal (hybrid meeting).
• Luetkemeyer AF, et al N Engl J Med. 2023;doi:10.1056/NEJMoa2211934.
• Mohseni M, Sung S, Takov V. Chlamydia. National Library of Medicine; 2019. https://www.ncbi.nlm.nih.gov/books/NBK537286/
• Molina J-M, et al. Abstract 119. Presented at: Conference on Retroviruses and Opportunistic Infections; Feb. 19-22, 2023; Seattle.
• National Institute of Allergy and Infectious Diseases. Safety and efficacy study of meningococcal group B vaccine rMenB+OMV NZ (Bexsero) to prevent gonococcal infection. ClinicalTrials.gov identifier: NCT04350138. Updated Nov. 24, 2023. Accessed Nov. 29, 2023. https://clinicaltrials.gov/study/NCT04350138.
• Petousis-Harris H, et al. Lancet. 2017;doi:10.1016/S0140-6736(17)31449-6.
• Springer C, Salen P. Gonorrhea. PubMed; 2021. https://www.ncbi.nlm.nih.gov/books/NBK558903/
• Stewart J, et al. Abstract 121. Presented at: Conference on Retroviruses and Opportunistic Infections; Feb. 19-22, 2023; Seattle.

For more information:

Marcus Hayner, PharmD, is a PGY1 pharmacy resident at Denver Health Medical Center.

Kati Shihadeh, PharmD, BCIDP, is a clinical pharmacy specialist in infectious diseases at Denver Health Medical Center. Shihadeh can be reached at [email protected].

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