“This new regimen is expected to lead to better TS inhibition”

Godefridus (Frits) J. Peters is professor emeritus at the Laboratory Medical Oncology, Amsterdam University Medical Center, professor at the Medical University of Gdansk, Poland, and honorary professor at Amity University in Noida, India. Frits Peters’ research focuses on the development of new cancer treatments, from preclinical research to clinical phase I and II studies. He has been involved in several clinical studies of drug candidates that later gained market approvals for the treatment of cancer and has been involved in the development of arfolitixorin for several years.

To begin with, what do you see as the greatest challenges in cancer treatment today?
— Cancer treatment has changed a lot in the last few decades, at least for several forms of cancer. To be more specific, this holds for several subforms of cancer, such as non-small cell lung cancer. This progress was achieved because of a better understanding of cancer biology and genetics. However, for several cancers, such as pancreatic ductal adenocarcinoma and malignant pleural mesothelioma, progress is still marginal, despite a better understanding of these diseases. This is due to late diagnosis, lack of effective treatment options and rapid spreading of the disease by forming metastases. Immunotherapy gives some benefit, but still for a limited number of patients.

Initial treatment of early-stage colorectal cancer is usually effective by using a combination of surgery and traditional chemotherapy. In advanced metastatic disease, fluoropyrimidine-based treatment options, like FOLFOX, in which 5-FU-based chemotherapy is combined with folinic acid (leucovorin) and oxaliplatin, are standard and form the core of almost all new protocols. For this group, the majority of patients do not survive beyond five years after diagnosis, so there is a high medical need.

You have played a central role in designing the ongoing phase Ib/II clinical study in metastatic colorectal cancer with arfolitixorin. From your perspective as an expert on folate anticancer treatments, what are the important and perhaps necessary changes that have been made compared to the phase III AGENT study?
—The efficacy of standard 5-FU-based chemotherapy depends on achieving an optimal and prolonged inhibition of its key target, thymidylate synthase (TS). In several historical studies, experimental and clinical, it was shown that the extent of TS inhibition and a prolonged maintenance of the same were related to better outcomes. When 5-FU is given alone, the active metabolite of 5-FU, FdUMP, forms an unstable binding, a binary complex with TS, resulting in poor and short-term TS inhibition. In standard regimens with 5-FU, leucovorin is administered as a prodrug for the active folate intermediate, 5,10-methylene-tetrahydrofolate (CH2-THF), which, together with FdUMP, forms a stable binding, a ternary complex with TS, which not only increases TS inhibition but also extensively prolongs TS inhibition. Arfolitixorin is the active form of [6R]-5,10-methylenetetrahydrofolate, which is the same molecule as CH2-THF. Unlike leucovorin, arfolitixorin does not need to be converted in the body to become active, but instead directly provides high levels of CH2-THF in tumor tissue.

In the AGENT study, 5-FU was administered about 30-60 minutes before arfolitixorin, and this most likely prevented optimal inhibition of TS, resulting in lower efficacy. Therefore, it was proposed to adapt the schedule and dose of arfolitixorin in order to maximize the extent and duration of TS inhibition. In the current phase Ib/II study, arfolitixorin is administered prior to 5-FU administration. This new regimen is expected to lead to a better TS inhibition, since upon 5-FU administration, the more effective ternary complex between FdUMP, TS and CH2-THF is immediately formed.

What scientific evidence supports the dosing regimen applied in the current phase Ib/II study?
— Preclinical studies in experimental models already showed that the extent and duration of TS inhibition were significantly improved with a longer duration when leucovorin was administered before 5-FU. In the Modelle-001 study, an investigator-lead clinical phase II trial, TS inhibition was systematically investigated in liver metastases from colorectal cancer patients. TS inhibition was compared in tumors from patients receiving a standard leucovorin dose, and a low and a high arfolitixorin dose, all before 5-FU. This schedule allowed a rapid accumulation of CH2-THF in the tumor, while the CH2-THF concentration was much lower in normal liver. Most importantly, the concentration of CH2-THF was higher in patients treated with low doses of arfolitixorin compared to those who received an equimolar dose of leucovorin. However, in patients treated with high doses of arfolitixorin, the accumulation of CH2-THF was even higher. Since 5-FU was administered after arfolitixorin, this allowed the immediate formation of the ternary complex in the tumor cells.

The most important finding was that TS inhibition was higher in the arfolitixorin-treated patients, and even higher in patients who received a higher dose. Based on, among other things, these data, the dosing and scheduling were changed in the phase Ib/II protocol; arfolitixorin is administered prior to 5-FU, allowing rapid formation of the stable ternary TS complex. This better and prolonged TS inhibition will not only enhance the formation of eg. oxaliplatin-DNA adducts, that triggers cell death, but also prevent the repair of these toxic DNA adducts.

Assuming a successful development of arfolitixorin, how do you envision this next-generation folate impacting cancer treatment in clinical practice?
— Earlier preclinical and clinical data showed a better antitumor effect (either evaluated as response or survival) when prolonged TS inhibition can be maintained. Since pretreatment with arfolitixorin followed by 5-FU leads to increased and prolonged TS inhibition, this is most likely leading to a better therapeutic efficacy. Recent data from a tumoroid model (patient-derived colon cancer organoids) also showed that a high arfolitixorin concentration leads to more cell death. So, I expect that the dual modulation – TS inhibition and oxaliplatin-DNA adducts – will lead to better efficacy than what has been observed with other recent protocols.