Radiosensitizers, Bioreductive drugs, Radioprotectors

1. Lecture 24 Radiosensitizers, Bioreductive drugs, Radioprotectors

2. Tumor radiosensitization • Halogenated pyrimidines, nitroimmidazoles • Hypoxic cell cytotoxins: tirapazamine • Normal tissue radioprotection • Mechanisms of action, sulfhydryl compound, • WR series, dose reduction factor (DRF) • Biological response modifiers

3. Tumor radiosensitization • Radiosensitizers are chemical or pharmacological agents that • increase the lethal effects of radiation if administered in • conjunction with it. Many compounds that modify the • radiation response do not show a differential effect • between tumors and normal tissues. • There is no point in employing a drug that increases the • sensitivity of tumor and normal cells to the same extent. • Only two types of sensitizers have found practical use in • clinical radiotherapy: • The halogenated pyrimidines • Hypoxic cell sensitizers

4. Tumor radiosensitization

5. Tumor radiosensitization • Halogenated pyrimidines, nitroimmidazoles • Hypoxic cell cytotoxins: tirapazamine • Normal tissue radioprotection • Mechanisms of action, sulfhydryl compound, • WR series, dose reduction factor (DRF) • Biological response modifiers

6. The Halogenated Pyrimidines The halogenated pyrimidines are very similar in structure to DNA precursos thymidine, having a halogen substituted for the methyl group. This “weakens” the DNA chain and makes the cells more susceptible to damage by gamma-rays or UV light. These substances are effective as sensitizers only if they are made available to cells for several cell generations, so that the analogue may be incorporated into the DNA. As the percentage of thymidine bases replaced increases, so does the extent of radiosensitization.

7. The Effectiveness of the Halogenated Pyrimidines

8. Tumor radiosensitization • Halogenated pyrimidines, nitroimmidazoles • Hypoxic cell cytotoxins: tirapazamine • Normal tissue radioprotection • Mechanisms of action, sulfhydryl compound, • WR series, dose reduction factor (DRF) • Biological response modifiers

9. Hypoxic Cell Radiosensitizers A search was under way in the early 1960s for compounds that mimic oxygen in their ability to sensitize biologic materials to the effects of X-rays. The approach: use oxygen substitutes that diffuse into poorly vascularized areas of tumors. They are not metabolized by the cells in the tumor as rapidly as oxygen. Because of this, they can penetrate further than oxygen and reach all of the hypoxic cells in tumor. Properties essential for a clinically useful hypoxic cell sensitizer: selective tumor cell sensitization and acceptable toxicity to the normal cells; Chemically stable; Highly soluble in water or lipids and capable of diffusing a considerable distance

11. Hypoxic Cell Radiosensitizers The first candidate to satisfy these criteria was Misonidazole

12. Hypoxic Cell Radiosensitizers

13. Hypoxic Cell Radiosensitizers Misonidazole produces appreciable sensitization with cells in culture

14. Hypoxic Cell Radiosensitizers Misonidazole also has a dramatic effect on tumors in experimental animals

15. Hypoxic Cell Radiosensitizers

16. Hypoxic Cell Radiosensitizers Spurred by the promise of misonidazole in the laboratory compared with its failure in the clinic, efforts were made to find a better drug

17. Hypoxic Cell Radiosensitizers • An alternative approach to designing drugs that are preferentially • radiosensitize hypoxic cells is to develop drugs that selectively • kill hypoxic cells. They are compounds that can be reduced • preferentially to cytotoxic species in the hypoxic regions of tumors. • Three classes of agents in this category are known: • The quinone antibiotics (Mitomycin C) • Nitroaromatic compounds (dual function agents) • The benzotriazine di-N-oxides (tirapazamine)

18. Hypoxic Cell Cytotoxins Tirapazamine This compound is believed to be activated by the enzyme cytochrome p450

19. Hypoxic Cell Cytotoxins Tirapazamine A transplanted mouse carcinoma was treated with X-rays alone, drug alone or a combination of the two The effect of the combination is much greater than additive

20. Hypoxic Cell Cytotoxins Tirapazamine The effect was even more dramatic in vivo scoring regrowth delay

21. Markers of hypoxic cells A major development in the past two decades has been the synthesis of radioactive-labeled nitroimidazoles for use as markers of hypoxic cells. Under conditions of reduced oxygen tension, the drug is metabolized, and broken down. In this study patients were given a tritiated thymidine- labeled drug before tumor was removed.

22. Markers of hypoxic cells The interesting result of the study is that only four of nine patients had tumors with a significant proportion of hypoxic cells. Only melanoma and small-cell lung cancer appeared to contain a proportion of hypoxic cells that would prejudice the outcome of radiotherapy

23. Tumor radiosensitization • Halogenated pyrimidines, nitroimmidazoles • Hypoxic cell cytotoxins: tirapazamine • Normal tissue radioprotection • Mechanisms of action, sulfhydryl compound, • WR series, dose reduction factor (DRF) • Biological response modifiers

24. Normal tissue radioprotectors Some substances do not directly affect the radiosensitivity of cells but they may protect whole animals because they cause vasoconstriction or in some way upset normal processes of metabolism to such extent that the oxygen concentration in critical organs is reduced. Because cells are less sensitive to X-rays under hypoxia, this provides a radioprotection of the normal tissues. Examples: sodium cyanide, carbon monoxide, epinephrine, histamine and serotonine. Such compound are not really radioprotectors per se.

25. Normal tissue radioprotectors Sulfhydryl compounds True radioprotectors are the sulfhydryl compounds. The simplest is cysteine, a sulfhydryl compound containing a natural amino acid. Structure of cysteine Structure of cysteamine NH2 SH – CH2 – CH COOH SH – CH2 – CH2 – NH2

26. Normal tissue radioprotectors Sulfhydryl compounds, DRF Animals injected with cysteamine require doses of X-rays 1.8 times higher that control animals to produce the same mortality rate. This factor of 1.8 is called the dose-reduction factor, defined as: Dose of radiation in the presence of drug DRF = Dose of radiation in the absence of drug to produce a given level of lethality

27. Tumor radiosensitization • Halogenated pyrimidines, nitroimmidazoles • Hypoxic cell cytotoxins: tirapazamine • Normal tissue radioprotection • Mechanisms of action, sulfhydryl compound, • WR series, dose reduction factor (DRF) • Biological response modifiers

28. Normal tissue radioprotectors Mechanism of action • Most efficient radioprotectors have a • certain features in common: • a free SH group at one end of the • molecule, and • a strong basic function such as amine • or guanidine at the other end. • The mechanism of SH-mediated • cytoprotection include: • Free radical scavenging • Hydrogen atom donation to facilitate • direct chemical repair at sites of • DNA damage

29. Normal tissue radioprotectors Mechanism of action If the free radicals can be scavenged before they can interact with biologic molecules, the effect of radiation is reduced. The protective effect of sulfhydryl compound tends to parallel the oxygen effect, being maximal for sparsely ionizing radiations (e.g., X- or gamma- rays ) and minimal for densely ionizing radiations (e.g. low-energy alpha particles)

30. Development of more effective compounds Cysteine is a radioprotector, but it’s also toxic and induces nausea and vomiting at the dose levels required for radioprotection. A development program was initiated in 1959 by the US Army to identify and synthesize less toxic drugs. Over 4,000 compounds were synthesized and tested. The toxicity is reduced if: sulfhydryl groups are covered by a phosphate group:

31. Development of more effective compounds WR series The structures of three typical compounds out of more than 4,000 synthesized are shown in the table: WR-638 – cystaphos, protects only against sparsely ionizing radiations; WR-2721 – amifostine, protects blood-forming organs WR-1607 – structure similar to other two WRs. It’s much more effective but also very toxic (rat poison), so it’s not usable.

32. WR series WR-2721 – amifostine

33. WR series WR-2721 – amifostine

34. WR series WR-2721 – amifostine Mouse is given a radioactive-labeled amifostine. The tumor has not taken up the drug at all, whereas the bone marrow and salivary glands show high uptake of the drug

35. Tumor radiosensitization • Halogenated pyrimidines, nitroimmidazoles • Hypoxic cell cytotoxins: tirapazamine • Normal tissue radioprotection • Mechanisms of action, sulfhydryl compound, • WR series, dose reduction factor (DRF) • Biological response modifiers

36. Biological Response Modifiers Biological response modifiers, used to treat cancer exert their antitumour effects by improving host defense mechanisms against the tumor. They enhance the ability of the host to tolerate damage by toxic chemicals that may be used to destroy the cancer. They can target the pathways tumor cells use to curcumvent normal growth regulation and may inhibit signals that protect tumor cells from radiation damage thus acting as radiosensitizers. Therefore, an increased tumor cell response to radiation might be observed, despite of the lack of direct cytotoxicity of these drugs. A possible synergistic effect of biological modifiers with radiation treatment is the basis and subject of ongoing studies.

37. Biological Response Modifiers Biologic Response Modifiers (BRM), also called immunotherapy, is a type of treatment that mobilizes the body's immune system to fight cancer.  BRMs consist of vitamins, hormones, and other natural drugs. The examples include: » Ascorbic Acid Mixture» Vitamin E » Cimetidine » Selenium» Inteferon beta » Interleukin-2 (IL-2) » Melatonin» Low Dose Bromocriptine (Parlodel) » Vitamin C » Cyclical Ginadal Hormones

38. Biological Response Modifiers The therapy mainly consists of stimulating the immune system to help it do its job more effectively.  Biological response modifiers are substances that are able to trigger the immune system to indirectly affect tumors.  These include cytokines such as interferons and interleukins.  This strategy involves giving larger amounts of these substances by injection or infusion in the hope of stimulating the cells of the immune system to act more effectively.