Novel development of 5-aminolevurinic acid (ALA) in cancer diagnoses and therapy
Introduction
With the improvements in cancer diagnosis, there have been increasing numbers of patients whose tumors are identified at an earlier stage, resulting in a decreased incidence of metastatic disease. [1] Further, this also has resulted in fewer patients undergoing invasive and curative therapy. [2] Indeed, it is now possible to simultaneously diagnose a suspected cancerous lesion and conduct “curative” therapy, thereby resulting in improved survival and quality of life. [3] 5-Aminolevulinic acid (ALA) is a natural amino acid biosynthesized not only in animals, but also in plant mitochondria. Together with light or laser irradiation, it is used as a photosensitizer precursor as part of photodynamic diagnosis (PDD) and photodynamic therapy (PDT) to identify/kill tumor cells, resulting in a new strategy for cancer diagnosis and therapy [4].
The polymerization of 8 ALA monomers results in the synthesis of protoporphyrin IX (PpIX), which is critical to heme synthesis. [5], [6] PpIX preferentially accumulates in tumor cells resulting in the down-regulation of PpIX to heme, [7], [8] and, thus, is applicable to PDD and PDT as a photosensitizer. [9] ALA-induced PDD allows simultaneous visualization and treatment of malignant glioma, superficial bladder cancer, and other neoplastic diseases. ALA, in contrast to other photosensitizers, may be orally administered with less risk of phototoxicity as an endogenous product. [10] ALA has also been synthesized chemically; however, the resultant product is unstable. Recently, we developed a novel method to synthesize high yields of stable ALA by fermentation. [11] Studies of ALA-induced PDD and PDT in animal models have demonstrated a decrease in metastases and tumor recurrence supporting its clinical use. [12], [13], [14] In this review, the biological potential of heme metabolites is discussed as well as the mechanisms of PpIX targeting tumor cells. In addition, enhancement of ALA-induced PDT's therapeutic potential, through its ability to perturb the tumor microenvironment and via combination with other therapeutic modalities, such as hyperthermia and immunotherapy, is also suggested.
Section snippets
What is ALA?
As described above, ALA is an endogenous amino acid, which is usually synthesized from glycine and succinyl CoA in mitochondria. [5], [6] Following systemic administration, ALA in tumor cells is metabolized into PpIX, a photosensitizing porphyrin and is considered an endogenous photosensitizer. In addition, ALA is water soluble and may be administered locally, systemically, and orally. For clinical use, ALA has been chemically synthesized; however, it is unstable, and, the synthesis is
Biosynthesis and metabolite of heme
Heme consists of ferrous iron (Fe2+) and PpIX such that heme acts as a precursor for hemoglobin, myoglobin, cytochrome, cytochrome P450, catalase, and peroxydase. [23] For example, hemoglobin consists of heme and globins, and is able to transport oxygen to various tissues because of its ability to bind to oxygen. Research concerning the biosynthesis of porphyrin and heme was initiated by Radin in 1950, who demonstrated that all carbon (C) and nitrogen (N) in heme derive from acetic acid and
Biological activity of heme and its metabolites
In order to circumvent the harmful effects of heme-related physiological or pathological cell damage, such as hemolysis and rhabdomyolysis, mammals have evolved a specific scavenger system against the heme-induced formation of reactive oxygen species (ROS). The hemoglobin-binding haptoglobin (Hpt-Hb) and the receptor CD163, as well as the heme-binding hemopexin (Hx-hem) and the receptor low-density-lipoprotein (LDL)-related protein-1 (LRP-1 or CD91), are considered the primary scavenger
ALA-induced PDD and PDT
Clinical research into the response of malignant tumors to ALA administration was first reported with its use as a photosensitizer for PDT by Malik, et al., in 1987. [46] Fukuda, et al., reported that ALA uniquely up-regulated tumor accumulation of PpIX and displayed potential as a photosensitizer for PDT in cancer. [47] Since then, clinical research into the diagnostic and therapeutic potential of ALA has been ongoing. In 1997, Stummer, using an experimental brain tumor model, demonstrated the
Mechanism of PpIX accumulation in tumor cells by ALA treatment
As mentioned above, PDD and PDT with ALA exploit the accumulation of PpIX by tumor cells following ALA administration, but, how ALA induces the accumulation of PpIX in tumor cells has not been completely identified. One hypothesis is that PpIX is actively synthesized from ALA in tumor cells and is decreased through the enzymatic activity of ferrochelatase, or if the amount of Fe2+ iron is insufficient, down-regulating the metabolism of PpIX to heme resulting in PpIX accumulation. Although no
Strategies to further improve the therapeutic efficacy of ALA-induced PDT
Abels reported a significant reduction in red blood cell velocity and pO2 in tumors and surrounding tissue after ALA-induced PDT, which normalized after 24 h. [67] Thus, one reason for the poor therapeutic effect by ALA-induced PDT may be a lack of irreversible ischemia. In addition, PpIX synthesis in tumors has also been reported following ALA injection in intracellular spaces rather than the intravascular space resulting in a transient ischemia. Currently, ALA-induced PDT is limited to the
Conclusion
In this review, the immunopharmacological significance of heme and its metabolites is discussed, including the induction of antitumor immunity with the goal of inducing a complete response to ALA-induced PDT. We suggest that oxidative stress after PDT can restrict angiogenesis and the immunological tumor response in the microenvironment of tumors. Further, we posit that it is possible to induce specific antitumor immunity following PDT in combination with anti-angiogenic agents, hyperthermia,
Acknowledgements
The authors would like to thank Professor Dr. James E. Talmadge, University of Nebraska Medical Center, for his critical discussion and also thank Ms. Alice S. Cole for editing the manuscript.
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