Doxycycline

Evidence Level: L5

Predicted Indications: 10

Doxycycline

Pharmacist Assessment Report

Doxycycline: From Bacterial Infections to Punctate Epithelial Keratoconjunctivitis

One-Sentence Summary

Doxycycline is a tetracycline antibiotic originally used for infections such as bronchitis, bacterial pneumonia, acne, anthrax, gonorrhea and chlamydia-type infections. The TxGNN model predicts it may be useful for post-bacterial disorder (the highest-ranked prediction that has registered clinical trials), with 11 clinical trials supporting this direction, although most of them test infection prevention rather than a post-bacterial disorder as such. The top-scoring prediction, punctate epithelial keratoconjunctivitis, has only a single 1992 case report behind it.

Quick Overview

Item	Content
Original Indication	Bronchitis, acne, bacterial pneumonia, anthrax, gonorrhea and other bacterial infections (the Canadian licence records contain no indication text)
Predicted New Indication	Punctate epithelial keratoconjunctivitis
TxGNN Prediction Score	0.9994 (rank 1); 0.9989 (post-bacterial disorder)
Evidence Level	L5 for rank 1 (a single case report); L1 for post-bacterial disorder (as graded in the compiled data, though the indication fit is loose)
Canada Market Status	Marketed (marketed DINs: 15 of 17; dormant: 2)
Number of DINs	17 (marketed: 15)
Recommended Decision	Hold for rank 1; Proceed with Guardrails for post-bacterial disorder

Why is This Prediction Reasonable?

Detailed mechanism of action data is not available for this drug. Based on what is known, doxycycline is a broad-spectrum tetracycline antibiotic with proven efficacy against many bacterial infections, including chlamydia, gonorrhea, syphilis-related organisms, rickettsial infections and brucellosis. It is also known to have anti-inflammatory and matrix metalloproteinase (MMP)-inhibiting effects.

For punctate epithelial keratoconjunctivitis, the link is that doxycycline treats chlamydial conjunctivitis and may calm ocular-surface inflammation. The only supporting paper, however, is a 1992 report of two patients whose punctate keratitis persisted after chlamydial conjunctivitis had resolved. It offers no controlled evidence that doxycycline treats the keratitis.

For post-bacterial disorder, the registered trials mainly test doxycycline as post-exposure prophylaxis (doxy-PEP) to prevent bacterial sexually transmitted infections, plus prevention of relapsing fever and treatment of chlamydia-induced reactive arthritis. This is prevention of bacterial infection rather than treatment of a post-bacterial condition, so the disease label is only loosely mapped to the trials and the indication fit should be confirmed.

Clinical Trial Evidence

The rank 1 prediction (punctate epithelial keratoconjunctivitis) has no registered clinical trials. The table below shows trials linked to the rank 4 prediction, post-bacterial disorder.

Trial Number	Phase	Status	Enrollment	Key Findings
NCT00237016	Phase 2/3	Completed	140	Double-blind, placebo-controlled trial of post-exposure doxycycline to prevent tick-borne relapsing fever
NCT00351273	Phase 3	Completed	42	6-month combination antibiotics for chlamydia-induced reactive arthritis lasting more than 6 months
NCT03980223	Phase 4	Completed	641	Doxy-PEP to reduce gonorrhea, chlamydia and syphilis in PrEP users and men with HIV who have sex with men
NCT04050540	Phase 4	Completed	449	Doxy-PEP to reduce bacterial STIs among Kenyan women taking HIV PrEP
NCT01207739	Phase 4	Completed	280	Prolonged oral antibiotic strategies after IV ceftriaxone in persistent Lyme disease
NCT04067011	Phase 2	Completed	210	Drug-vaccine interaction study of AV7909 with ciprofloxacin or doxycycline (pharmacokinetics and immunogenicity); not directly about this indication
NCT06468462	Phase 4	Recruiting	2900	WHO-recommended periodic presumptive treatment vs doxy-PEP for STI control in men who have sex with men in Kenya
NCT04762134	Not applicable	Recruiting	560	Randomized trial of doxycycline chemoprophylaxis for STIs in gay, bisexual and other men who have sex with men (Canada)
NCT06069141	Not applicable	Recruiting	688	Single-dose benzathine penicillin G with or without doxycycline for early syphilis in people living with HIV
NCT07397403	Phase 4	Recruiting	400	Efficacy and safety of doxy-PEP against gonorrhea, chlamydia and syphilis in Bangkok

Literature Evidence

The rank 4 prediction has no linked literature. The table below shows the single paper behind the rank 1 prediction.

PMID	Year	Type	Journal	Key Findings
1424659	1992	Case report	Cornea	Two patients with chlamydial follicular conjunctivitis treated with tetracycline or doxycycline had resolution of follicles, then developed recurrent punctate epithelial keratitis

Canada Market Information

Seventeen single-ingredient licences are currently valid in Canada. The compiled data contain no dosage form or indication text for them. Five are listed below.

DIN	Product Name	Dosage Form	Approved Indication
02558505	NRA-DOXYCYCLINE	Not available	Not available
02247104	PMS-DOXYCYCLINE	Not available	Not available
02375885	APPRILON	Not available	Not available
02543478	M-DOXYCYCLINE	Not available	Not available
02512645	APO-DOXYCYCLINE MR	Not available	Not available

One further licence is DOXYCYCLINE FOR INJECTION USP (DIN 02561441), so an injectable form exists alongside the oral products.

Safety Considerations

Drug Interactions: A screening query returned 100 interacting drugs. The first 20 reviewed include these:
- Major: isotretinoin, acitretin, aminolevulinic acid
- Moderate: aluminum hydroxide, aminophylline, amoxicillin, ampicillin, bedaquiline, balsalazide, attapulgite and others
- Minor: acetazolamide, amiloride, bendroflumethiazide
Knowledge-graph contraindication signals (not label text): Clostridium difficile colitis, perinatal necrotizing enterocolitis, and dyskinesia of the esophagus.
Prediction-specific concerns:
- Otitis externa: an in vitro study shows tetracycline cytotoxicity in human tympanic membrane fibroblasts, a concern for ear use.
- Doxy-PEP: tetracycline resistance (for example in gonococci and S. aureus) and microbiome effects.
- Infection-related hemolytic uremic syndrome: antibiotics in Shiga toxin-producing E. coli infection may raise HUS risk.

Please refer to the package insert for the official warnings and contraindications.

Conclusion and Next Steps

Decision: Hold for punctate epithelial keratoconjunctivitis (rank 1); Proceed with Guardrails for post-bacterial disorder (rank 4)

Rationale:

The top-scoring prediction rests on a single 1992 case report with no trials, so it cannot support action yet.
Post-bacterial disorder has the strongest trial base (a completed Phase 2/3 trial, a completed Phase 3 trial and several Phase 4 trials). These trials mainly show prevention of bacterial infection, so the fit to a “post-bacterial disorder” is loose.
Other predictions are weaker: chronic gingivitis has many periodontitis studies but they concern a different condition (L3), while Chagas cardiomyopathy has a negative preclinical result and infection-related hemolytic uremic syndrome has no evidence at all.

To proceed, the following is needed:

Confirmation that the post-bacterial disorder label matches the conditions studied in the trials
Package insert warnings and contraindications from Health Canada, which are currently missing and block safety screening
Mechanism of action data
A guardrail plan covering antimicrobial resistance, microbiome effects and drug interactions (especially retinoids)
For gingivitis, verification of whether the periodontitis evidence applies, and for rank 1, controlled evidence in keratoconjunctivitis
Disclaimer

This content is for research purposes only and does not constitute medical advice. Clinical validation is required before any clinical application.