Idetrexed

Drug Overview

Idetrexed, formerly known by its experimental names BTG945, ONX-0901, and CT900, is an investigational oral Targeted Therapy developed for the treatment of solid tumors — with ovarian cancer as its most advanced and promising indication. It is a potent thymidylate synthase inhibitor that triggers cell death while selectively targeting the alpha folate receptor (FRα), which is highly expressed in cancer cells compared to normal tissues across a variety of solid tumors.

What sets idetrexed apart from older antifolate chemotherapy drugs is its precision. Rather than indiscriminately attacking all rapidly dividing cells — including healthy ones — idetrexed is engineered to be selectively delivered into cancer cells that carry a specific surface marker, making it a true Targeted Therapy rather than a broad cytotoxic agent.

• Generic Name: Idetrexed (formerly BTG945, ONX-0901, CT900)
• US Brand Name: None assigned — currently investigational
• Drug Class: Alpha Folate Receptor–Targeted Thymidylate Synthase Inhibitor / Antifolate / Targeted Therapy
• Route of Administration: Intravenous (IV) infusion (phase I and II studies)
• FDA Approval Status: Not FDA-approved. Idetrexed was created by scientific teams at the Institute of Cancer Research (ICR) Centre for Cancer Drug Discovery, London, UK. It has completed a first-in-human phase I trial and is now advancing into phase II clinical trials under development by US-based pharmaceutical company Algok Bio.

What Is It and How Does It Work? (Mechanism of Action)

To understand idetrexed, it helps to know what keeps cancer cells multiplying. All dividing cells — healthy or cancerous — need to build new DNA. A critical step in this process is the production of thymidine, one of DNA’s four building blocks. An enzyme called Thymidylate Synthase (TS) is responsible for manufacturing thymidine. Without it, cells cannot copy their DNA and cannot divide.

Idetrexed is a folate-receptor targeted TS inhibitor that attacks this process with remarkable precision. Here is how it works step by step:

Selective Receptor Targeting: Idetrexed selectively binds to the alpha folate receptor (α-FR), a cell-surface glycoprotein, with very high affinity similar to that of folic acid (KD ∼0.1 nmol/L). This receptor is abundant on cancer cells but has very restricted expression on healthy tissues — making idetrexed’s entry highly selective for tumor cells.

Receptor-Mediated Entry: Once bound to the α-FR on the cancer cell surface, idetrexed is pulled inside the cell through a process called receptor-mediated endocytosis — essentially the cell swallowing the drug by folding its membrane around it.

Thymidylate Synthase Inhibition: Once inside, idetrexed acts as a potent inhibitor of thymidylate synthase (TS; Ki TS = 1.4 nmol/L), leading to cell death. By blocking TS, idetrexed cuts off the cancer cell’s supply of thymidine, making DNA replication impossible and forcing the cell into programmed death (apoptosis).

Sparing Normal Tissues: The α-FR has restricted expression in TS-responsive normal proliferating tissues such as gut and bone marrow, and this expression is limited to the apical surface membrane and therefore not accessible to FR-binding agents in the circulation. This is what makes idetrexed fundamentally different from older antifolates — it causes far less damage to healthy cells.

FDA Approved Clinical Indications

Oncological Uses (Under Clinical Investigation — Not FDA-Approved):

• Ovarian cancer — primary investigational focus; over 90% of ovarian cancer cases express FRα, with high levels of overexpression also observed in endometrial, triple-negative breast cancer, and mesothelioma. Kidney, lung, colorectal, and gastric cancers also exhibit varying degrees of expression.

Non-Oncological Uses:

• There are currently no non-oncological indications under investigation for idetrexed.

Dosage and Administration Protocols

Idetrexed is administered intravenously in a clinical trial setting. Based on phase I data, the recommended phase II dose has been identified. As an investigational drug, all dosing is conducted under strict clinical trial protocols by qualified oncology teams.

Clinical Efficacy and Research Results

Idetrexed has generated compelling early-phase clinical evidence that has driven its advancement into phase II trials.

A phase I study conducted by the ICR and The Royal Marsden treated 109 cancer patients, including ovarian cancer patients resistant to platinum-based chemotherapy, with idetrexed. Among 25 platinum-resistant ovarian cancer patients with high or medium expression of FRα who received the recommended phase II dose, tumor shrinkage was observed in nine patients, representing an overall response rate of 36% (95% CI, 18–57.5%).

This is a clinically remarkable result. Platinum-resistant ovarian cancer is one of the most difficult-to-treat cancers in oncology, with most approved therapies producing response rates in the 10–15% range. An ORR of 36% in this population positions idetrexed as a potentially significant advance.

Professor Udai Banerji, Principal Investigator of the idetrexed phase I study, noted that idetrexed has shown comparable efficacy to drugs such as antibody drug conjugates that have gone on to successful late phase development. Algok Bio will soon initiate pivotal studies for registering idetrexed for ovarian cancer treatment, alongside additional clinical programmes exploring other indications and combination regimens with standard care.

Safety Profile and Side Effects

Black Box Warning: There is no FDA Black Box Warning for idetrexed, as it is not FDA-approved. Based on phase I data and the known profile of thymidylate synthase inhibitors in this class, the following safety profile has been established.

Common Side Effects (>10%):

• Nausea and Vomiting — most commonly reported across all TS inhibitors in this class; typically grade 1–2 and manageable with standard antiemetics
• Fatigue — common; generally mild and does not typically require dose modification
• Myelosuppression — low white blood cell and platelet counts observed at higher dose levels in phase I; monitor CBC regularly
• Elevated Liver Enzymes — asymptomatic, reversible transaminase elevations observed; monitor LFTs during treatment

Serious Adverse Events:

• Grade 3/4 Thrombocytopenia: Reported at higher dose levels in the phase I escalation study; requires prompt dose interruption and platelet monitoring
• Serious Infections: Risk increases during periods of neutropenia; any fever must be evaluated immediately
• Embryo-Fetal Toxicity: All antifolate agents carry the theoretical risk of fetal harm due to interference with DNA synthesis; pregnancy is contraindicated

Management Strategies:

• For nausea: administer prophylactic antiemetics before each infusion; maintain oral hydration
• For myelosuppression: monitor CBC at baseline and before each cycle; hold treatment for grade 3/4 events
• For elevated LFTs: monitor at baseline and during treatment; discontinue if significant elevation persists

Connection to Stem Cell and Regenerative Medicine

A key scientific advantage of idetrexed’s folate receptor–targeted delivery mechanism is its significantly reduced toxicity to bone marrow stem cells compared to conventional antifolates. The α-FR has restricted expression in TS-responsive normal proliferating tissues such as gut and bone marrow, meaning idetrexed largely spares the stem cell niche. This property makes it a candidate for combination with other myelosuppressive therapies — including platinum-based chemotherapy and emerging immunotherapy regimens — without the additive bone marrow damage seen with older TS inhibitors. Ongoing research is exploring idetrexed in combination with standard-of-care agents for platinum-resistant ovarian cancer, where preserving bone marrow function is critical to tolerating long-term treatment.

Patient Management and Practical Recommendations

Pre-Treatment Tests:

• FRα expression testing of tumor tissue — mandatory; idetrexed is only active in FRα-expressing tumors
• Complete blood count (CBC) with differential and platelet count
• Liver function tests (LFTs) and kidney function panel (eGFR/creatinine)
• Pregnancy test for all women of childbearing age — negative result required before enrollment

Precautions During Treatment:

• CBC and LFTs must be monitored before each treatment cycle
• Patients must be enrolled in an approved clinical trial to access idetrexed — it is not available outside of trial settings
• Avoid folate supplementation during treatment unless specifically directed by the treating physician, as excess folic acid may compete with the drug for receptor binding

Do’s and Don’ts:

• DO attend all scheduled infusion appointments and follow-up visits without exception
• DO report any fever, unusual bruising, or bleeding to your care team immediately
• DO inform your care team of all medications and supplements — including over-the-counter vitamins containing folic acid
• DON’T become pregnant during treatment — idetrexed carries a theoretical risk of fetal harm
• DON’T take idetrexed outside of an approved and supervised clinical trial program
• DON’T stop participation in the trial without discussing with your oncologist first

Legal Disclaimer

The information in this guide is for educational purposes only and does not constitute medical advice, diagnosis, or a treatment recommendation. Idetrexed is an investigational drug that is not approved by the US Food and Drug Administration (FDA) or any major regulatory authority for routine clinical use. It is currently available only through participation in approved clinical trials. All treatment decisions must be made in consultation with a qualified oncologist or licensed healthcare professional with full knowledge of the patient’s complete medical history. This content does not replace official investigational protocol documentation or professional medical judgment.