8.2 Pharmacotherapy of Sexually Transmitted Infections

Pharmacotherapy of Sexually Transmitted Infections

Sexually transmitted infections (STIs) represent an escalating clinical and public health challenge characterized by changing antimicrobial susceptibility patterns. Contemporary CDC STI Treatment Guidelines emphasize high-dose targeted bactericidal therapies, the elimination of regimens prone to resistance selection, and two-step resistance-guided approaches for emerging pathogens.

---

Neisseria gonorrhoeae Infections

Neisseria gonorrhoeae is a fastidious Gram-negative diplococcus infecting mucosal columnar and transitional epithelial cells of the urethra, endocervix, rectum, pharynx, and conjunctiva. Over several decades, N. gonorrhoeae has acquired resistance to every therapeutic class deployed against it, including sulfonamides, penicillins, tetracyclines, and fluoroquinolones.

Uncomplicated Gonorrhea Regimen (CDC STI Guidelines)

In late 2020, the CDC officially dismantled the historical dual-therapy regimen (ceftriaxone 250 mg IM + azithromycin 1 g PO) due to escalating azithromycin resistance among gonococcal isolates and the imperative to conserve macrolides. Ceftriaxone monotherapy at doubled dosing is now the standard of care:

                       Suspected or Confirmed Neisseria gonorrhoeae
                                            │
                     ┌──────────────────────┴──────────────────────┐
                     ▼                                             ▼
           Cephalosporin Tolerant                       Severe Cephalosporin Allergy
                     │                                  (Anaphylaxis / Angioedema)
         ┌───────────┴───────────┐                                 │
         ▼                       ▼                                 ▼
   Weight < 150 kg        Weight ≥ 150 kg               Gentamicin 240mg IM single dose
   Ceftriaxone 500mg IM   Ceftriaxone 1g IM                           PLUS
   single dose            single dose                   Azithromycin 2g PO single dose
         │                       │                                 │
         └───────────┬───────────┘                                 │
                     │                                             │
                     ▼                                             ▼
          Chlamydia Ruled Out?                           Test of Cure Required?
          • Yes: No additional abx                       • Urogenital/Rectal: No TOC needed
          • No / Pending: Add Doxycycline                  (unless persistent symptoms)
            100mg PO BID x 7 days                        • Pharyngeal: MANDATORY TOC at
                                                           7 to 14 days via NAAT or culture

Critical Clinical Nuances for Gonococcal Infections

1. Pharmacokinetic Rationale for High-Dose Ceftriaxone: Elevated minimum inhibitory concentrations (MICs) to cephalosporins (mediated by mosaic penA alleles encoding altered penicillin-binding protein 2 [PBP2]) require higher systemic and mucosal drug exposures. A 500 mg intramuscular dose achieves serum concentrations exceeding the MIC90 of resistant strains for $> 24$ hours, reliably clearing mucosal colonization.
2. Pharyngeal Gonorrhea Challenges and Test of Cure (TOC):
   • Gonococcal infection of the pharynx is predominantly asymptomatic and exhibits significantly lower microbiological eradication rates than urogenital or rectal infections due to lower drug penetration into oropharyngeal lymphoid tissue and salivary secretions.
   • Test of Cure (TOC) is MANDATORY for all patients with pharyngeal gonorrhea. TOC should be performed 7 to 14 days after treatment completion using culture or nucleic acid amplification testing (NAAT). If NAAT is positive at 7–14 days, confirmatory culture with antimicrobial susceptibility testing must be performed immediately.
   • For urogenital and anorectal gonorrhea treated with first-line ceftriaxone, routine TOC is not recommended if symptoms resolve; however, all patients must be retested at 3 months post-treatment to detect reinfection from untreated sexual partners.
3. Disseminated Gonococcal Infection (DGI):
   • Results from bacteremic dissemination of strains resistant to serum bactericidal action (typically carrying specific PorB1A porin variants).
   • Triad / Clinical Presentation: Tenosynovitis (wrists, fingers, ankles), dermatitis (painless pustular or hemorrhagic acral papules), and migratory asymmetrical polyarthralgia; or purulent monoarticular septic arthritis (knee, ankle, wrist).
   • Inpatient Regimen: Ceftriaxone 1 g IV every 24 hours for at least 7 days (extend to 10–14 days for gonococcal meningitis and $\ge 4$ weeks for endocarditis). Step-down to oral cefixime 400 mg PO BID can be considered 24 to 48 hours after substantial clinical improvement if susceptibility is confirmed.

---

Chlamydia trachomatis Infections

Chlamydia trachomatis (serovars D through K) is an obligate intracellular bacterium that infects squamocolumnar epithelial cells. It represents the most frequently reported bacterial infectious disease in the United States.

Treatment Paradigm Shift: Doxycycline vs. Azithromycin

Historically, azithromycin 1 g PO single dose and doxycycline 100 mg PO BID for 7 days were considered therapeutically equivalent. However, landmark randomized controlled trials and meta-analyses established that doxycycline is clinically and microbiologically superior to azithromycin, particularly for extragenital (anorectal) infections:

Special Populations: Pregnancy and Infant Prophylaxis

• Pregnancy: Doxycycline is contraindicated during the second and third trimesters of pregnancy due to permanent intrinsic tooth discoloration (yellow-gray-brown banding), enamel hypoplasia, and transient suppression of fetal long-bone growth.
  • Preferred First-Line Regimen in Pregnancy: Azithromycin 1 g orally as a single dose.
  • Alternative Regimen in Pregnancy: Amoxicillin 500 mg PO three times daily for 7 days.
  • Test of Cure (TOC) Mandate: All pregnant patients treated for chlamydia must undergo a test of cure via NAAT 4 weeks after completion of therapy to verify microbiological eradication and prevent transmission during vaginal delivery.
• Neonatal Conjunctivitis and Pneumonia: Caused by peripartum exposure to infected maternal cervical secretions. Infant chlamydial conjunctivitis or pneumonia is treated with oral erythromycin ethylsuccinate (50 mg/kg/day divided 4 times daily for 14 days) or oral azithromycin (20 mg/kg/day daily for 3 days). Both agents are associated with infantile hypertrophic pyloric stenosis (IHPS), mandating clinical monitoring for non-bilious projectile vomiting.

---

Syphilis (Treponema pallidum)

Syphilis is a chronic systemic infection caused by the spirochete Treponema pallidum subspecies pallidum. Its clinical course progresses through primary, secondary, latent, and tertiary stages, with neurosyphilis, ocular syphilis, and otosyphilis potentially developing at any point in the infection continuum.

Clinical Staging and Manifestations

1. Primary Syphilis: Characterized by a painless, indurated ulcer (chancre) at the site of inoculation, accompanied by regional, non-tender, rubbery lymphadenopathy. Resolves spontaneously within 3 to 6 weeks.
2. Secondary Syphilis: Spirochetal dissemination occurring 2 to 8 weeks after chancre emergence. Characterized by a non-pruritic maculopapular rash prominently involving the palms and soles, condylomata lata (flat, moist, highly infectious verrucous plaques in intertriginous folds), mucous patches, generalized lymphadenopathy, and patchy "moth-eaten" alopecia.
3. Latent Syphilis: Seroreactivity in the absence of clinical manifestations.
   • Early Latent: Infection acquired within the preceding 12 months (documented seroconversion, 4-fold titer increase, or history of primary/secondary lesions or contact with a confirmed case).
   • Late Latent / Unknown Duration: Infection acquired $> 12$ months prior, or where date of acquisition cannot be established.
4. Tertiary Syphilis: Manifests decades after untreated infection: cardiovascular syphilis (ascending aortic aneurysm, coronary ostial stenosis, aortic regurgitation) or gummatous lesions (granulomatous destructive lesions of skin, bones, and viscera).
5. Neurosyphilis, Ocular Syphilis, and Otosyphilis: Can emerge during any stage. Early forms include syphilitic meningitis, cranial nerve palsies, uveitis, and sudden sensorineural hearing loss. Late forms include general paresis (progressive dementia, delusions) and tabes dorsalis (posterior column spinal cord degeneration causing sensory ataxia and lancinating pains).

Pharmacotherapeutic Regimens by Stage

Penicillin Allergy and Mandatory Desensitization in Pregnancy

• The Absolute Rule for Syphilis in Pregnancy: There is no acceptable alternative to parenteral penicillin G for the treatment of maternal syphilis. Penicillin G reliably crosses the placenta, eradicates spirochetes in the maternal bloodstream, clears fetal hepatic and hematologic infection, and prevents devastating congenital syphilis (hydrops fetalis, stillbirth, Hutchinson teeth, saddle nose, saber shins).
• Mandatory Protocol: Any pregnant patient with a reactive syphilis serology and a documented history of penicillin allergy must undergo formal penicillin allergy skin testing. If skin testing is positive (or history confirms severe IgE-mediated anaphylaxis), the patient must be hospitalized for oral or intravenous penicillin desensitization, followed immediately by administration of stage-appropriate Benzathine penicillin G.
• Alternatives in Non-Pregnant Patients: Doxycycline 100 mg PO BID for 14 days (early syphilis) or 28 days (late latent). Ceftriaxone 1 to 2 g IV/IM daily for 10 to 14 days is an alternative, but cross-reactivity must be weighed in severe anaphylaxis.

The Jarisch-Herxheimer Reaction

• Pathophysiology: An acute, systemic inflammatory reaction triggered by the rapid, mass bactericidal lysis of Treponema pallidum following the first dose of treponemicidal antimicrobial therapy. Disrupted spirochetes release high quantities of membrane-bound lipoproteins (such as TPP47), which activate host Toll-like receptors (TLR2/TLR4) on macrophages and dendritic cells. This causes an explosive systemic surge of pro-inflammatory cytokines: tumor necrosis factor-alpha (TNF-$\alpha$), interleukin-6 (IL-6), and interleukin-8 (IL-8).
• Clinical Presentation: Onset occurs within 2 to 24 hours following penicillin administration. Manifestations include abrupt high fever, shaking rigors, tachycardia, tachypnea, hypotension, myalgias, headache, flushing, and transient exacerbation of secondary skin lesions.
• Clinical Management:
  • The Jarisch-Herxheimer reaction is benign, self-limiting, and universally resolves within 12 to 24 hours.
  • Management is purely symptomatic with antipyretics (acetaminophen, ibuprofen) and intravenous hydration.
  • Critical Patient Counseling: Patients must be reassured that this reaction is NOT an allergic reaction to penicillin and does not represent therapeutic failure or contraindicate completion of therapy.
  • Pregnancy Warning: In pregnant females treated for secondary syphilis, the reaction can provoke uterine contractions, premature labor, and transient fetal heart rate decelerations. Patients in the second half of pregnancy should be educated and monitored in an obstetric setting during initial treatment.

Serologic Response and Post-Treatment Monitoring

Syphilis staging and therapeutic efficacy are tracked through two distinct serologic testing modalities:

1. Treponemal Tests (FTA-ABS, TP-PA, CIA, MFI): Detect antibodies directed against specific Treponema pallidum surface antigens. Once positive, treponemal assays remain reactive for life ("serofast") in $\ge 85–90\%$ of patients regardless of successful treatment. They cannot distinguish active disease from successfully treated historical infection.
2. Non-Treponemal Tests (RPR, VDRL): Quantitative assays measuring IgG and IgM antibodies directed against cardiolipin-lecithin-cholesterol antigens released from damaged host cells and treponemes. Non-treponemal titers correlate directly with disease activity and inflammatory burden.
3. Definition of Treatment Success: A four-fold decline in non-treponemal titer—equivalent to a decrease of two dilution steps (e.g., a decline from 1:32 to 1:8, or from 1:64 to 1:16).
4. Monitoring Schedule:
   • Primary and Secondary Syphilis: Non-treponemal titers evaluated at 6 and 12 months post-therapy.
   • Latent Syphilis: Non-treponemal titers evaluated at 6, 12, and 24 months.
   • HIV-Infected Individuals: Evaluated more frequently at 3, 6, 9, 12, and 24 months.
   • Treatment Failure Criteria: Failure to achieve a 4-fold titer decline within 12 to 24 months, or a sustained 4-fold titer increase (e.g., from 1:4 to 1:16), defines therapeutic failure or reinfection. This mandates lumbar puncture for CSF analysis to rule out neurosyphilis, followed by retreatment with Benzathine penicillin G 2.4 million units IM weekly for 3 consecutive weeks.

---

Genital Herpes Simplex Virus (HSV) Infections

Genital herpes is a chronic, lifelong viral infection caused by herpes simplex virus type 1 (HSV-1) or type 2 (HSV-2). Following primary mucocutaneous infection, the virus ascends sensory nerve axons to establish lifelong latent infection within dorsal root sacral ganglia.

Antiviral Mechanism of Action

All approved anti-herpetic agents are nucleoside analogs:

• Acyclovir: Synthetic purine deoxynucleoside analog of guanine. Selectively monophosphorylated by viral thymidine kinase (present only in HSV- and VZV-infected cells) into acyclovir monophosphate. Host cellular kinases subsequently synthesize acyclovir diphosphate and the active acyclovir triphosphate. Acyclovir triphosphate competitively inhibits viral DNA polymerase and, lacking a 3'-hydroxyl group, acts as an obligate chain terminator of elongating viral DNA strands.
• Valacyclovir: The L-valyl ester prodrug of acyclovir. Rapidly and nearly completely converted to acyclovir via first-pass hepatic metabolism by valacyclovir hydrolase, yielding oral bioavailability of 55% (compared to 15–20% for oral acyclovir), permitting less frequent daily dosing.
• Famciclovir: Diacetyl ester prodrug of 6-deoxy-penciclovir. Converted to penciclovir triphosphate, which acts as a competitive inhibitor of viral DNA polymerase.

Clinical Management Regimens (CDC STI Guidelines)

---

Trichomonas vaginalis Infections

Trichomonas vaginalis is a flagellated, anaerobic, parasitic protozoan that infects the squamous epithelium of the lower urogenital tract.

Diagnostic and Pharmacotherapeutic Updates (CDC STI Guidelines)

• Clinical Paradigm Shift: The historical standard of metronidazole 2 g PO single dose was updated based on multi-center randomized controlled trials demonstrating marked clinical superiority of a 7-day multi-dose course in women.
• Recommended Regimen for Females:
  • Metronidazole: 500 mg orally twice daily for 7 days.
  • Evidence: In an open-label RCT among women with trichomoniasis, 7-day metronidazole reduced repeat infection rates at 1 month by nearly half compared to the 2 g single dose (11% vs. 19%, relative risk 0.55).
• Recommended Regimen for Males:
  • Metronidazole: 2 g orally as a single dose remains an acceptable alternative regimen for men.
• Alternative Regimen:
  • Tinidazole: 2 g orally as a single dose. Tinidazole possesses a longer elimination half-life (~12–14 hours vs. 8 hours for metronidazole), achieves higher tissue concentrations, and produces fewer gastrointestinal side effects.
• Pharmacological Interaction: Alcohol Avoidance:
  • Metronidazole and tinidazole inhibit hepatic aldehyde dehydrogenase, impairing the oxidation of acetaldehyde into acetate. Ingestion of ethanol during therapy precipitates a disulfiram-like reaction characterized by severe facial flushing, pounding headache, diaphoresis, nausea, intractable vomiting, chest pain, and palpitations.
  • Patients must strictly avoid all alcoholic beverages and ethanol-containing medications during treatment and for at least 24 hours following the final metronidazole dose (and at least 72 hours following tinidazole due to its prolonged half-life).

---

Mycoplasma genitalium

Mycoplasma genitalium is a minute, slow-growing, wall-less bacterium that is an established cause of recurrent non-gonococcal urethritis (NGU) in males, cervicitis and pelvic inflammatory disease in females, and tubal infertility.

Resistance Biology and Two-Step Resistance-Guided Therapy

Due to its lack of a peptidoglycan cell wall, M. genitalium is intrinsically resistant to all beta-lactams, glycopeptides, and fosfomycin. Resistance to macrolides (point mutations in the 23S rRNA gene, predominantly A2058G and A2059G) and fluoroquinolones (mutations in parC and gyrA) has escalated dramatically worldwide, with macrolide resistance exceeding 50% to 70% in many urban centers.

                           Diagnosis of Mycoplasma genitalium via NAAT
                                                │
                                                ▼
                               Doxycycline 100mg PO BID x 7 days
                       (Reduces bacterial load and clears coinfections)
                                                │
                     ┌──────────────────────────┴──────────────────────────┐
                     ▼                                                     ▼
        Macrolide-Susceptible Isolate                         Macrolide-Resistant Isolate
         (or Macrolide Testing Sensitive)                     (or Resistance Testing Unavailable)
                     │                                                     │
                     ▼                                                     ▼
         High-Dose Azithromycin                                 Moxifloxacin 400mg PO
         1g PO day 1, followed by                               daily x 7 days
         500mg PO daily on days 2–4
         (Total dose: 2.5 grams)

1. Step 1 (Bacterial Load Reduction): All patients receive Doxycycline 100 mg PO twice daily for 7 days. Doxycycline alone achieves low microbiological cure rates (30–40%), but it suppresses bacterial replication, lowers the target organism burden, and prevents the emergence of secondary resistance mutations during subsequent therapy.
2. Step 2 (Antimicrobial Eradication Guided by Resistance Testing):
   • Macrolide-Susceptible Strains: Follow doxycycline immediately with High-Dose Azithromycin: 1 g orally on day 1, followed by 500 mg orally once daily on days 2 through 4 (total cumulative azithromycin dose: 2.5 g).
   • Macrolide-Resistant Strains (or if resistance testing is unavailable): Follow doxycycline immediately with Moxifloxacin 400 mg orally once daily for 7 days. Moxifloxacin retains potent topoisomerase activity and achieves $> 85–90\%$ cure in macrolide-resistant isolates.

---

Doxycycline Postexposure Prophylaxis (doxy-PEP)

CDC issued its first formal doxy-PEP clinical guideline in 2024 (MMWR Recomm Rep 2024;73(RR-2)), making bacterial STI chemoprophylaxis a routine pharmacist counseling topic.

• Eligible population: Men who have sex with men and transgender women who have had syphilis, chlamydia, or gonorrhea diagnosed in the preceding 12 months, after shared decision-making. CDC does not recommend it for other populations, where trial evidence is lacking or negative.
• Regimen: Doxycycline 200 mg orally as a single dose within 72 hours after oral, vaginal, or anal sex, not to exceed 200 mg every 24 hours. It is patient-managed and episodic, not scheduled daily dosing.
• Efficacy: Randomized trials reduced syphilis and chlamydia by more than 70% and gonococcal infection by roughly 50%.
• Counseling and monitoring: Take with water and remain upright to limit esophagitis; expect photosensitivity and gastrointestinal upset; continue STI screening every 3 to 6 months plus HIV testing; reassess need at least annually. Monitoring for tetracycline resistance in N. gonorrhoeae and S. aureus is an explicit stewardship responsibility.
• Distinguish the three prophylaxis strategies an exam vignette may blend: doxy-PEP (bacterial STIs, after sex), HIV PEP (28-day three-drug regimen, within 72 hours), and HIV PrEP (ongoing, before exposure).

---

Expedited Partner Therapy (EPT)

Expedited Partner Therapy (EPT) is the evidence-based clinical practice of treating the sex partners of patients diagnosed with confirmed chlamydia, gonorrhea, or trichomoniasis by providing prescriptions or medications directly to the patient to deliver to their partner, without requiring the healthcare clinician to perform an in-person physical examination of the partner.

Clinical and Legal Principles of EPT

• Legal Landscape: EPT is legally permissible in the vast majority of U.S. states and territories. Antimicrobial stewardship and infectious diseases pharmacists play a critical role in facilitating EPT protocols within hospital emergency departments, public health clinics, and ambulatory practices.
• Primary Clinical Objective: Interrupts transmission chains, prevents reinfection of the index patient, and reduces population-level disease incidence.
• Standard EPT Prescribing Regimens:
  • Chlamydia EPT: Doxycycline 100 mg PO twice daily for 7 days (or single-dose Azithromycin 1 g PO if partner adherence to a 7-day course is doubtful).
  • Gonorrhea EPT: Because the first-line regimen requires intramuscular ceftriaxone which cannot be self-administered, the recommended oral EPT regimen for gonorrhea is Cefixime 800 mg orally as a single dose PLUS Doxycycline 100 mg orally twice daily for 7 days (to cover potential chlamydia coinfection).
  • Trichomoniasis EPT: Metronidazole 500 mg PO twice daily for 7 days (or 2 g PO single dose).
• Mandatory EPT Educational Packaging: Every EPT medication or prescription must be accompanied by comprehensive written educational materials that detail:
  1. The specific STI being treated and the necessity of treatment.
  2. Clear warnings regarding drug allergies, contraindications, and potential adverse effects.
  3. Strict warning against taking the medication if pregnant, with instructions to seek immediate prenatal medical care.
  4. Counseling to avoid all sexual contact until at least 7 days after both partners have completed therapy and all symptoms have resolved.