A Phase I Trial of a Methionine Restricted Diet with Concurrent Radiation Therapy

Abstract

Methionine is an essential amino acid critical for cell growth and survival. Preclinical evidence suggests a methionine restricted diet (MRD) sensitizes cancer to radiation therapy (RT), without significant adverse effects. However, this has never been evaluated in humans. The purpose of this pilot study was to evaluate the safety and feasibility of concurrent MRD with standard-of-care definitive RT in adults with any non-skin cancer malignancy. The MRD extended from 2 wk before RT initiation, through 2 wk beyond RT completion. The primary endpoint of safety was assessed as rate of grade 3 or higher acute and late toxicities. Feasibility was assessed with quantitative plasma amino acid panel every 2 wk during the MRD (target plasma methionine 13 μM). Nine patients were accrued over a two-year period, with five able to complete the treatment course. The trial was closed due to slow accrual and subjects’ difficulty maintaining the diet. No grade 3 or higher adverse events were observed. Subjects’ average methionine level was 18.8 μM during treatment, with average nadir 16.8 μM. These findings suggest the safety of concurrent MRD with RT, with toxicities comparable to those expected with RT alone. However, the diet was challenging, and unacceptable to most patients.

Introduction

Chemotherapy is commonly used as a radiosensitizer to improve the efficacy of definitive radiation therapy (RT), though at the expense of increased toxicity (1). Therefore, development of alternative approaches to radiosensitization that improve the therapeutic ratio is of great interest. One of the hallmarks of cancer is the uncontrolled growth and spread of cells (2). For rapid and continuous proliferation to occur, synthesis of new molecules, including DNA and proteins, is necessary. Protein synthesis in humans requires the amino acid methionine to initiate translation, as well as to serve as a building block during polypeptide synthesis. Methionine is also used in DNA methylation, affecting gene expression, as well as the synthesis of glutathione, a free radical scavenger (Figure 1) (3, 4). Despite its critical roles, internal methionine synthesis is limited, and therefore must be supplementarily acquired via diet (4).

Figure 1.

Biochemical cycle of methionine (from Epner et al. (17)).

Normal cells are able to tolerate a methionine deficient environment; however, during tumorigenesis, most cells become methionine auxotrophs, reliant on dietary intake of methionine for growth and survival, which is known as the Hoffman effect (5). This reliance may potentially be exploited clinically for better treatment outcomes. By limiting dietary methionine intake, protein synthesis is slowed due to a decreased ability to initiate translation, leading to slowed cell cycle progression. This makes cancer cells especially vulnerable because of their higher proliferative rates compared to normal cells. Additionally, the resulting decreased glutathione levels lead to an increased oxidative environment, which can lead to cell cycle arrest and cell death (6–8). Multiple preclinical studies have demonstrated that methionine restriction diets (MRDs) may enhance chemotherapy treatment (9, 10). Recently, there have also been preclinical studies demonstrating radiosensitization through methionine modulation (11, 12). Overconsumption of dietary methionine, as is common in the Western diet, has also been associated with higher radiation toxicity in mice (13).

In theory, a temporary dietary modification may be appealing to patients if it enables avoidance of cytotoxic chemotherapy as a radiosensitizer, however, the combination of the MRD and radiation therapy has not been previously evaluated in human subjects. Therefore, this Phase I trial was conducted to evaluate whether a MRD/radiation combination therapy is feasible and safe in patients receiving curative-intent radiation therapy for their malignancy (clinical trial ID # NCT03574194).

Materials and Methods

Patient Criteria

Adult patients were screened for participation if they had non-skin malignancy to be treated with curative intent definitive or adjuvant RT alone (without concurrent chemotherapy), and an RT course duration of 2–6 wk. There were no restrictions based on the body site treated, or whether treatment involved 3-dimensional conformal radiation therapy (3DCRT), intensity-modulated radiation therapy (IMRT), or stereotactic body radiation therapy (SBRT). Standard of care dose-volume constraints were used for radiation treatment planning. Other inclusion criteria included Zubrod performance status of 0–2; adequate bone marrow and organ function; negative plasma or urine pregnancy test within 72 h of registration for women of child-bearing age; and ability to give consent for trial participation.

Exclusion criteria included the following: prior radiation therapy which would overlap with current radiation fields; unintentional weight loss of greater than 10% body weight in the past six months; use of an appetite stimulant as treatment for weight loss within the previous six months; cytotoxic chemotherapy within four weeks prior to beginning treatment; planned local therapy or cytotoxic chemotherapy within four weeks of treatment completion; active pericardial, pulmonary, or systemic infection; psychiatric or social difficulties that would result in difficulty complying with study requirements; pregnant or lactating women; and soy allergy, due to the methionine-free medical food supplements containing soy products.

Methionine Restricted Diet

Personalized meal plans were developed in conjunction with a clinical dietician to meet a goal of 5–10 mg/kg body weight/day methionine consumption, while maintaining adequate protein and caloric intake. The diet consisted of low-protein cereals, grains, and breads; fruits; vegetables; margarines and oils; and simple carbohydrates. Patients were advised to avoid foods with high methionine content, such as meats, fish, and most cheeses. The diet is similar to a vegan diet, with some additional restrictions on intake of specific nuts/legumes that are high in methionine content. A commercially available methionine-free protein supplement (HC Cooler 15, HCU Express 15, or HCU Express 20 [Vitaflo]) was also provided to patients free of charge, in an unlimited supply, to minimize weight loss and hunger. Examples of food containing low, medium, and high amounts of methionine per serving is given in Table 1. The target methionine plasma level was 13 μM, based on prior clinic data from Durando et al (14). Subjects kept a log of their dietary intake, and reviewed it every 2 wk with the clinical dietician to promote adherence to the diet. Subjects were also offered a $100 food voucher to compensate for any additional food costs that may have been involved in adhering to the experimental diet.

Table 1.

Examples of foods with varying methionine content.

Low (up to 75 mg/serving)	Medium (76–205 mg/serving)	High (>206 mg/serving)
Asparagus- 1 cup	Corn- 1 cup	Beef
Broccoli- 1 cup	Peas- 1 cup	Chicken
Carrots- 1 cup	Spinach- 1 cup	Ham
Cauliflower- 1 cup	Tofu, Medium Firm- ½ cup	Turkey
Kale- 1 cup	Mung Beans- 1 cup	Fish
Mushrooms- 1 cup	Refried Beans- 1 cup	Shellfish
onions- 1 cup	Soy Burgers- 1 patty	Yogurt
Sweet Potato- 1 cup	Peanuts, Dry Roasted- 1 oz	Cheese- Cheddar, Cottage, etc.
Pumpkin- 1 cup	Pistachios- 1 oz	Brazil Nuts
Bananas- 1 cup	Cashews- 1 oz	Mixed Nuts
Grapes- 1 cup	Eggs-large
Peaches- 1 cup	Whole Milk- 1 cup
Plums- 1 cup	Brie, Gouda Cheeses- 1 oz
oranges- 1 cup	Pumpkin Seeds- 1 oz
Cantaloupe- 1 cup	Sunflower Seeds- 1 oz
Soy Milk, Fortified- 1 cup	Wheat Pasta- 1 cup
Saltine or Ritz Crackers- 8 crackers	White Rice- 1 cup
Barley- 1 cup	Quinoa- 1 cup
Almonds- 1 oz	Brown Rice- 1 cup
Pecans- 1 oz

Treatment and Analysis

The experimental design is illustrated in Figure 2. The MRD was initiated two weeks before beginning RT, since the majority of the decrease in plasma methionine concentration takes place in this time period (15). The MRD extended through two weeks beyond completion of RT. The primary objective was to determine the safety of the combination therapy, using a primary endpoint of the rate of grade 3 or higher acute and late toxicities per Common Terminology Criteria for Adverse Events (CTCAE) version 5.0, over 12 months follow-up. During the follow-up period, patients underwent history and physical exam, with directed imaging or laboratory workup as indicated per standard of care for their cancer type, every 3 months. The secondary objective of feasibility was assessed using a dietary log of food intake, and with a quantitative plasma amino acid panel during the MRD at 2–3week intervals after initiation. Exploratory endpoints included local control and disease-free survival, as assessed using Response Evaluation Criteria in Solid Tumors (RECIST) version 1.1 criteria. The target accrual was 15 subjects. Descriptive statistics are reported. The protocol was reviewed and approved by the West Virginia University Institutional Review Board (approval number 1803047835), and registered on ClinicalTrials.gov (NCT03574194).

Figure 2.

Experimental design.

Results

Over two years, 53 patients were offered enrollment, and 9 subjects enrolled; 5 successfully completed the MRD and RT, while 4 withdrew during the induction phase of the MRD prior to starting RT. The trial was closed early due to slow accrual and subjects’ difficulty maintaining the diet. Table 2 summarizes subjects’ characteristics and outcomes. All subjects completed the 12-month follow-up period, over which time there were no grade 3 or higher adverse events attributable to the MRD, and no episodes of disease recurrence. Methionine plasma levels varied over the course of treatment, and while no subject achieved the target of 13 μM, two nadired at 14 μM. The average nadir for all 5 subjects was 16.8 μM methionine, and their average methionine level while on treatment was 18.8 μM.

Table 2.

Patient characteristics and outcomes.

Subject	Age	Sex	RT Technique and Site Treated	Total RT Dose (Gy)	Dose/Fraction (Gy)	Methionine Nadir (μM) and (Mean Level on Treatment)	Toxicities (Grade)
1	82	Male	SBRT to Stage I lung cancer	54	18	19 (28)	Pneumonitis (G1)
2	67	Female	SBRT to Stage I lung cancer	55	11	19 (23)	Dyspepsia (G1) Fatigue (G2)
3	67	Female	SBRT to 2 oligometastatic lung nodules	39	13	14 (16)	Wheezing (G1)
4	53	Female	IMRT to pelvic and para-aortic nodes from recurrent endometrial cancer	45/60	1.8/2.4 (integrated boost)	18 (23)	Diarrhea (G2) Weight Loss (G2)
5	65	Male	IMRT to pelvic nodes and post-prostatectomy tumor bed	45/66.6	1.8 (sequential boost)	14 (19)	None

Discussion

Preclinical evidence suggests that restricting dietary methionine intake has the potential to improve clinical outcomes in patients with cancer. However, prior clinical trials utilizing a methionine restricted diet for up to 39 wk, either alone or in combination with cytotoxic chemotherapy, have shown poor feasibility due to failure to meet patients’ energy needs, resulting in side effects such as weight loss, leukopenia, and neutropenia (14–16). Our pilot study combined RT with a potentially radiosensitizing MRD for the first time in humans, with the premise that utilizing the experimental diet for a relatively short duration may be more feasible for patients to complete. The findings suggest that the MRD was safe with thoracic or abdominopelvic RT, with toxicities comparable to those expected with RT alone. However, the diet was not desirable for most patients who were offered the trial and refused, and was not acceptable for most of the patients enrolled, who either withdrew from participation, or had difficulties adhering to the diet enough to reduce their plasma methionine levels to the target. These limitations led to a small patient pool for analysis.

Our findings demonstrate the challenges of major dietary changes, even for a short period of time, which was perhaps made more challenging due to the concomitant effects of radiation. The geographic location at which the trial was conducted (West Virginia) may have also contributed, as enrollment may have been improved in a region with a higher prevalence of patients who naturally follow a vegetarian or vegan diet. If methionine restriction is to be utilized as a radiosensitizer in the future, methods to facilitate compliance will be critical to successful implementation. This may be potentially accomplished by making the methionine-free food supplement more palatable to patients (17), or creating a diet kit for patients that would provide all of their nutritional intake for the duration of their treatment course. Current National Cancer Institute dietary guidelines for patients with cancer recommend high protein and calories for improved healing, decreased infection, and to mitigate side effects such as anorexia and taste changes, so a diet kit centered around methionine restriction would need to have a suitable substitute for high protein foods like meats and eggs (18). Several novel approaches are also under investigation for treatment of inborn cystathionine beta-synthase-deficient homocystinuria, for which a MRD is effective but is similarly poorly adhered to (19). For example, one such approach allows patients to consume a more normal diet, with prescription of an oral pharmaceutical designed to deplete plasma methionine levels (20, 21).

Conclusion

While the number of participants who successfully completed the trial limits the strength of possible conclusions, this study suggests the safety of a combination use of MRD and radiation therapy. Further developments in feasible implementation of a MRD may support future clinical trials evaluating its radiosensitizing potential.

Data Availability Statement

Research data are stored in an institutional repository and will be shared upon request to the corresponding author.