Amino acid deprivation vs. mainstream cancer therapeutics: exploring the potential and limitations

Cancer is the primary cause of death on a global scale, necessitating ongoing research and treatment developments. The primary mechanism that distinguishes cancerous cells from healthy ones is metabolism. Metabolism is the fundamental requirement for living things to sustain vital processes and preserve equilibrium. Any type of change in this metabolic homeostasis, referred to as metabolic dysregulation, can lead to the development of many disorders, including cancer. Cancer cell metabolic profiling revealed that these rapidly growing cells employ specific metabolic strategies that optimize biomass and energy production [1]. In this context, metabolic regulation of tumor immunity has emerged as a prominent area of research, particularly following the success of immune checkpoint – based immunotherapy. Several metabolic pathways are now being explored as therapeutic targets, including glycolysis (especially in combination with immune checkpoint inhibitors), amino acid metabolism, lipid metabolism and adenosine signaling [2,3]. These approaches are increasingly being pursued for their potential to enhance anti-tumor immunity and improve therapeutic outcomes. Targeting metabolism has thus evolved into an effective method for combating cancer.

Cancer cells require nutrients like amino acids more than healthy cells do because of their increased proliferation rate. These cells rely on absorbing many amino acids from the environment and have increased expression of amino acid transporters [4]. Interestingly, under various oncogenic stimuli, several metabolic pathways typically active in healthy cells are often disrupted in malignant cells [5]. For instance, oncogenic stimuli in numerous cancer types make them auxotrophic to specific amino acids. Recognizing this reliance, researchers are looking into the possibility of deliberately reducing the accessibility of such amino acids for cancer cells, thereby impeding their proliferation and endurance. Targeting these metabolic addictions can involve reducing the number of amino acids in the environment, restricting transporter uptake, or lowering the activity of proteins and enzymes involved in their metabolism [1,4,6,7].

Arginine, asparagine, glutamine, lysine, methionine, phenylalanine, and tyrosine are among the amino acids that have been identified as potential targets for cancer treatment. These amino acids are vital for the growth and metabolism of tumor cells, playing a critical role in their overall function. Thus, various drug candidates have been developed to use this treatment approach; some of them have either been introduced into clinical practice or are undergoing clinical evaluation. L-asparaginase (ASNase) was the first molecule to be approved. The history of ASNase commenced in 1922 when the asparaginolytic activity of guinea pig (GP) serum was reported, although its origins were not identified. A significant discovery was made 30 years later by Kidd, who discovered that treatment with GP serum (but not horse or rabbit serum) caused malignant lymphoma in mice to regress rapidly [8]. The anticancer characteristics of GP serum were initially linked to ASNase activity, which was not an unexpected discovery. This finding spurred more investigation, which resulted in the identification of ASNase as the serum’s active component in 1961 and the investigation of ASNase’s possible anticancer properties [9]. The first commercially available L-asparaginase was Asparaginase (Elspar), an L-asparaginase II derived from E. coli, which received FDA approval in 1978 for treating pediatric acute lymphoblastic leukemia (ALL). Since then, several formulations have been introduced, including pegaspargase (Oncaspar), the first pegylated L-asparaginase approved in 2006, and asparaginase erwinia chrysanthemi (recombinant)- rywn (Rylaze), approved in 2021. Beyond its established efficacy in ALL, L-asparaginase has also demonstrated activity against acute myeloid leukemia (AML) and a range of solid tumors, such as glioblastoma, breast, pancreatic, and hepatocellular carcinomas. However, further characterization of ASNase variants and the development of personalized combination therapies are necessary to maximize its therapeutic potential [10]

Since ASNase has been performing successfully for over four decades, heightened importance has been placed on amino acid deprivation therapy (AADT), which has led to several studies being conducted in this area. Arginase and arginine deiminase are enzymes used to deplete arginine and are currently in development for various cancers. ADI-PEG 20, a PEGylated form of arginine deiminase, is being investigated in combination with other chemotherapeutic agents. Moreover, multiple variants of arginase 1 (BCT-100, PT-01, and AEB1102) are also under clinical development and have been found to be effective in the treatment of various cancers. Different forms of Methioninase, such as the nonrecombinant form from Pseudomonas putida and the tumor therapeutic bacterium SGN1, are also in clinical trials [1].

While L-asparaginase has been licensed and used successfully, the field of cancer therapy has come a long way in the last several decades, and despite all of this, it should be noted that, after L-asparaginase, not a single AADT medication has been granted FDA approval. However, in the same span of time, other therapies have grown dramatically. For instance, the approval of muromonab-CD3 (Orthoclone OKT3) for acute allograft rejection in 1986, started the field of immunotherapy [11]. Thereafter, a significant turning point in the history of the treatment of non-Hodgkin lymphoma was reached in 1997 with the approval of Rituximab, the first anti-cancer antibody ever developed. As of 2023, more than 200 therapeutic antibodies or antibody-based therapeutics have been approved [12], >50 of which were approved for the treatment of cancer [13]. Like antibodies, chemotherapy has also grown significantly throughout this period, and these anticancer medications are currently dominating the cancer therapy market. For instance, with the FDA approval of the first kinase inhibitor (KI), imatinib mesylate (Gleevec), for the treatment of myeloid leukemia in 2001, there are now 80 approved KI’s as of December 2023, 69 of which are used to treat cancer [14]. Affordability concerns are still present for some of these medications, as evidenced by the price of antibodies, which are more expensive than antibodies used to treat other conditions and are reported to cost over $100,000 a year when used for cancer treatment [15]. Thus, despite the fact that they are inexpensive and effective as well as the research on AADT is encouraging, the reason why there hasn’t been a successful novel agent approved for clinical use development in over 45 years deserves serious consideration.

The initial lack of understanding regarding the pharmacokinetic (PK) and pharmacodynamic (PD) profiles of these amino acid-depleting enzymes in patients may have contributed to the slow advancement of these molecules. Additionally, some enzymes exhibit nonspecific activity; for example, ASNases derived from E. coli and Erwinia also demonstrate glutaminase activity, which has been associated with adverse effects such as thrombosis, immunosuppression, and neurological complications [16]. Furthermore, the heterologous origin of most of these enzymes remains a notable drawback. Due to their microbial origin, these enzymes have a relatively short circulatory half-life and are immunogenic to humans. However, modification approaches viz. PEGylation have solved PK/PD issues of many therapeutics to a large extent, including recombinant enzymes. PEGylated asparaginase and arginase variants are examples of how polyethylene glycol (PEG) can be conjugated to an enzyme, addressing the issues of immunogenicity and toxic effects, which were once thought to be the primary constraints of these treatments. Also, due to their human origin, arginase variants have a significant advantage over other enzymes since they exhibit low immunogenicity [17]. However, it could possibly be argued that the lack of approval of PEGylated AADT may have to do with the time-consuming, expensive, and precise definition required for the conjugation of PEG to molecules. However, other strategies, such as the use of fusion proteins with half-life extension partners (HLEP), have also been investigated and proven to be successful. Furthermore, to be clear, PEGylation is still being used and is thought to be an effective technique. Researchers have been successfully using several PEGylated biologics, including PEGylated ASNase. Therefore, it cannot be implied that the lack of progress in the development and approval of the AADT therapeutics is due to a problem with any of these protein engineering techniques [18].

Although leading candidates such as arginase, arginine deiminase, and Methioninase have shown promising efficacy in various preclinical and clinical stages, their clinical translation has been hindered by several barriers. In arginine auxotrophic cancers, metabolic plasticity allows tumors to develop resistance through re-expression of enzymes like argininosuccinate synthase 1 and ornithine transcarbamylase or by upregulating alternative pathways and transporters [18,19]. Moreover, enzymes such as arginine deiminase and Methioninase are of non-human origin, making them highly immunogenic and leading to the generation of neutralizing antibodies, such as anti-METase antibodies [20]. Another challenge is that not all cancers are susceptible to the deprivation of a single amino acid, which limits the scope of these therapies. AADTs tend to be most effective when combined with other chemotherapeutic agents [18], but this requirement adds complexity to trial design and execution [4]. Additionally, anticancer enzymes typically have a narrow spectrum of activity, necessitating the identification of prognostic biomarkers to predict tumor susceptibility [13]. Developing and implementing such biomarkers across large patient populations involves significant financial and logistical challenges. Furthermore, the efficacy of AADT is influenced not only by cancer-intrinsic factors but also by external elements such as tumor location, the tumor microenvironment, and metabolic crosstalk with immune cells. Immune cells, particularly T cells, play a critical role in tumor suppression, and several amino acids, including arginine, glutamine, methionine, and cysteine, are essential for their proliferation, activation, and effector function. AADT may inadvertently impair these immune responses by limiting the availability of such amino acids [17]. Collectively, these factors may explain the limited clinical success and lack of regulatory approvals for new AADT agents, despite their mechanistic promise and long-standing interest in the field.

Even with all these drawbacks, enzyme-based amino acid depleting therapies remain highly appealing since, in comparison to other anticancer medications, they are affordable, safe, and easily producible compared to other therapeutics. Researchers are still investigating and studying AADT due to these appealing factors, but even after the first AADT molecule was approved almost half a century ago, we are unable to license another molecule from the same category, while other players who entered the market later are now dominating and growing at a significant rate. Although immunotherapies and kinase inhibitors have been shown to be promising, it is worth considering why, despite prior exploration and development, AADT drugs were not developed at the same pace. It is evident that there is no issue with this therapy technique, given the numerous recent studies that have also demonstrated its efficacy [1,6,7]. So, is it conceivable that we are not giving this strategy more serious consideration since other pharmaceuticals have made tremendous advancements in the fight against cancer, putting them in the forefront of the market and drawing our attention away from AADT?

Funding Statement

This paper was not funded.

Article highlights

• L-Asparaginase remains the sole clinically approved amino acid deprivation therapy despite decades of research into alternatives like arginase, methioninase, and glutaminase.

• Promising preclinical candidates face persistent clinical translation hurdles, including toxicity profiles, metabolic resistance mechanisms, and formulation challenges.

• In contrast, mainstream cancer therapies such as immunotherapy and tyrosine kinase inhibitors have progressed rapidly during the same period.

Author contributions

Abhay H. Pande: Conceptualization, Writing, review & editing. Yenisetti Rajendra Prasad: Data curation, Writing – original draft, Writing – review & editing. J. Anakha: Data curation, Writing – original draft, Writing, review & editing.

Disclosure statement

The authors have no relevant affiliations or financial involvement with any organization or entity with a financial interest in or financial conflict with the subject matter or materials discussed in the manuscript. This includes employment, consultancies, honoraria, stock ownership or options, expert testimony, grants or patents received or pending, or royalties.