# Implications for methenamine hippurate use in recurrent urinary tract infection management: Formaldehyde resistance and altered urinary composition

## TL;DR

Methenamine hippurate, used to prevent UTIs, may lead to bacterial resistance due to formaldehyde adaptation, highlighting the need for careful use and mindset change.

## Contribution

The study identifies formaldehyde resistance in clinical E. coli isolates and altered urine composition linked to methenamine hippurate use.

## Key findings

- ALTAR urinalysis detected formaldehyde in 85% of MH users' urine.
- E. coli isolates showed growth in high formaldehyde concentrations, indicating resistance.
- MH users had elevated formate levels in urine, suggesting altered metabolism.

## Abstract

Methenamine is a urinary antiseptic used to prevent urinary tract infections (UTI) via conversion to formaldehyde in the urinary tract. Methenamine hippurate (MH) is non-inferior compared to antibiotic (ABX) prophylaxis to manage recurrent UTI (rUTI) as demonstrated in the clinical trial ALTAR. Treatments such as MH, can improve antibiotic stewardship, as the primary treatment option for UTI is antibiotics. However, MH exhibits an elevated incidence risk with respect to breakthrough UTI as defined during ALTAR. Formaldehyde is highly toxic, while also a common byproduct of cellular metabolism. Powerful detoxification pathways exist to overcome formaldehyde toxicity. One example is the thiol-dependent metabolism of formaldehyde to formate in bacteria. The urinalysis of ALTAR urines detected formaldehyde in 85% of participant urines who were taking MH. HPLC analysis of a subset of urines from MH and ABX ALTAR participants, determined a significant change in urine composition. This included elevated levels of formate in urines from MH users. The thiol-dependant formaldehyde detoxification system of Escherichia coli is encoded by the frmRAB operon. The genes frmAB encode the enzymes responsible for detoxification, while frmR encodes a repressor of the system. ALTAR derived E. coli isolates were screened for growth in the presence of formaldehyde with 5.8% able to grow in > 1 mM formaldehyde. Bioinformatics identified 4 frmR alleles encoding non-functional FrmR variants and two plasmid-encoded frmA homologues. Growth in artificial urine confirmed that E. coli was susceptible to methenamine-formaldehyde conversion at pH6.0 and 5.6. All strains encoding frmR alleles grew in the presence of > 1 mg/ml methenamine at pH 5.6. The identification of FDHR in a clinical context and the changes in urine composition can improve the managed use of MH. However, a mindset change is needed to accept that MH, like antibiotics, has its own associated risks, including bacterial resistance.

The mainstay of treatment for urinary tract infections is short course antibiotics, often sufficient to clear the infection. However, a significant proportion of UTIs are recurrent, leading to frequent antibiotic consumption. The drive for better awareness of antibiotic prescription and antibiotic stewardship initiatives has led to research into alternatives to antibiotics for both management and prevention. The clinical trial ALTAR has determined that an alternative to antibiotic prophylaxis, methenamine hippurate, is not inferior with respect to improving the prevention of recurrent UTI episodes. Methenamine hippurate acts by releasing formaldehyde into urine through the breakdown of methenamine in acidic conditions generated by hippurate. The assumed mode of action is therefore that formaldehyde acts as a urinary antiseptic, and that as it is not an antibiotic, resistance cannot develop. Our data argues that uro-associated Escherichia coli is adapting to exposure to formaldehyde leading to methenamine hippurate resistance. Methenamine hippurate has its merits to improve antibiotic stewardship when treating recurrent UTI patients. However, a mindset change is needed by healthcare practitioners and patients to accept that, like antibiotics, there are associated risks, including bacterial resistance.