Introduction:

Background: Prostate cancer cells are especially sensitive to folate depletion. More recently the major folate receptor (FolH1) on prostate cancer, also called Prostate Specific Membrane Antigen (PSMA), has been targeted not only for imaging metastatic prostate cancer but for radiopharmaceutical therapy. PSMA is normally suppressed by testosterone but is upregulated in periods of androgen deprivation. While this interaction has been previously observed, the timing of the inverse relationship between androgen and the folate receptor has not specifically been targeted. 

Earlier phase clinical data: Preliminary data has shown an association between folate elevations and androgen deprivation therapy (ADT). A retrospective study at the veteran’s hospital in San Antonio examined 56 men (40 men on maintenance ADT; 16 initiating ADT) with previous folic acid measurements (Liss et al, 2020; PMID: 32444177). This revealed that 15 of 16 men initiating ADT had significant increases of their serum red blood cell (RBC) folate (compared to the maintenance ADT group, p=0.04) with more rapid time to mortality if folate levels >200 ng/ml. Overall survival was approximately 5 years if folate levels >200 ng/mL and was unreached in those with stable folate levels (p=0.02).

Our group then examined the effectiveness and feasibility of a low folate diet to reduce folic acid in a cohort of men with low-risk prostate cancer on active surveillance (Ullevig et al, 2021; PMID: 34330478) . A significant reduction of dietary folic acid and subsequently in serum folate and RBC folate was obtained, but compliance with the diet was challenging, raising questions on long term feasibility as an intervention.

Another potential method of reducing folic acid is pharmacologically. Trimethoprim acts as a dihydrofolate reductase inhibitor and is frequently prescribed as a prophylactic antibiotic (100 mg once daily (OD) or as 160 mg OD when combined with sulfamethoxazole) in patients with a history of recurrent urinary tract infections. A double blind, placebo controlled, randomized controlled trial demonstrated that serum folate was significantly decreased (p = 0.018) in patients randomized to trimethoprim 200 mg twice daily for 7 days, with a mean decrease in serum folate among trimethoprim exposed of 1.95 nmol/L, compared with a 0.21 nmol/L mean increase in the placebo group (P = 0.040) (Meidal Peterson et al, 2016; PMID: 26566149) . No serious adverse events were observed.

Hypothesis: We hypothesize that the use of trimethoprim 150 mg OD for 3 months will act as a folic acid inhibitor at the time of androgen deprivation, where expression of folate receptors is increased.

Objectives: The primary objective is safety of trimethoprim 150 mg OD for 3 months, using the National Cancer Institute (NCI) Common Terminology Criteria for Adverse Events (CTCAE v5). Secondary objectives include examining if trimethoprim can reduce RBC/serum folate, trimethoprim’s effect on prostate cancer biomarkers (prostate specific antigen (PSA) and urine PSMA), and changes in gut microbiome associated with trimethoprim.

Methods:

Study design and intervention: This study is a phase II, open label study (NCT06536374) randomizing patients starting ADT for prostate cancer to trimethoprim 150 mg OD for 3 months + standard of care (SOC) versus SOC alone. Randomization will occur between groups in a 1:1 ratio using computer pseudorandom numbers in random blocks of size 2, 3, or 4. The criteria were met for an Investigational New Drug exemption (IND:169666) after review by the US Food and Drug Administration. Figure 1 outlines the study schema, and the study schedule is described in Table 1.

Our primary outcome investigates safety, which does not need a sample size calculation. However, based on the discussed trial examining trimethoprim for folate reduction, folic acid levels were reduced in 83% versus 43% of patients on trimethoprim and placebo respectively. Therefore, with an alpha of 0.05 and allowing for a 10% dropout rate, 25 patients will be required in each arm (50 patients total) to achieve 80% power.

Enrollment status: Open, but not yet recruiting

Inclusion criteria: Patients with proven prostate cancer who are commencing ADT for any indication will be included (e.g., intermediate/high risk localized prostate cancer starting ADT+ radiotherapy, metastatic hormone-sensitive prostate cancer treated with or without androgen receptor pathway inhibitor with exception of abiraterone). Patients must also have a normal complete blood count, RBC folate >750nmol/L, homocysteine <15mcmol/L and normal creatine clearance.

Exclusion criteria: Patients will be excluded if commencing chemotherapy or if they underwent previous ADT/immunotherapy/chemotherapy in the last 2 years. Concomitant interventions that would warrant exclusion include those that interfere with folate metabolism (e.g., methotrexate), and abiraterone (in view of hyperkalemia) among others. Patients with known hematological disorders, allergy/hypersensitivity to trimethoprim and/or recurrent urinary tract infections will be excluded.

Outcome measurements and statistical analysis:

Primary outcome: Safety will be compared by evaluating occurrence of adverse events as per the NCI CTCAE v5 and adherence to trimethoprim based on drug diary logs and pill counts.

Secondary outcomes: RBC and serum folate will be measured at baseline and 1, 3 and 6 months after initiation of ADT.  Folic acid intake will be assessed via the National Health and Nutrition Examination Survey Dietary Screener Questionnaire 2009-10.

Serum PSA and urine PSMA levels will be measured at baseline and 1, 3 and 6 months after initiation of ADT. Serum PSA and testosterone will also be measured at 12, 18 and 24 months as part of SOC visits to determine 2- year progression-free survival.

Rectal swabs and stool samples will undergo longitudinal microbiome analysis at baseline and 1, 3 and 6 months. Stool will undergo microbiome (16s RNA, V1-V2 primers) and metabolomic evaluation (via metagenome prediction using PICRUSt bioinformatics package) to investigate within sample and between sample microbial diversity as well as mechanisms of bacterial folic acid biosynthesis. Alpha and beta-diversity analyses will be performed using QIIME. The Mann-Whitney U test will be employed to evaluate beta-diversity distances within and between groups of interest.

Results:

Conclusion:

Funding: Mays Cancer Center Pilot Funding