thiouracil and Melanoma

Various aspects of radiotherapeutic techniques used in treating ocular tumors are discussed. Previous and current "standard" modalities are briefly reviewed, including beta and gamma emitters, 60Co, 125I, and charged particles. In particular, emphasis is placed upon techniques under development that use biomolecules to physiologically target radioactive or stable isotopes to ocular melanoma. These procedures include 35S-thiouracil, 127I-deoxyuridine in conjunction with irradiation with photons from 145Sm, neutron capture therapy, and various combinations of the above. Thiouracil shows particular promise because of its incorporation during malanin synthesis in growing melanoma. Radiosensitization and stimulation of Auger cascades via introduction of iodine in DNA followed by irradiation with activating photons of appropriate energy (from 145Sm) has been shown to be effective in vitro. Various molecules may be used to transport boron to ocular melanoma, including thiouracil, thus allowing selective irradiation of tumor cells via 10B (n, alpha) 7Li reaction.

Topics: Antibodies, Monoclonal; Choroid Neoplasms; Cobalt Radioisotopes; Deoxyuridine; Dose-Response Relationship, Radiation; Eye Neoplasms; Helium; Humans; Iodine Radioisotopes; Melanins; Melanoma; Methods; Ophthalmology; Protons; Radioisotopes; Thiouracil

Thiocarbamides were converted to their di-N-pentafluorobenzyl (PFB) derivatives and analysed by gas chromatography/negative-ion chemical-ionization mass spectrometry with methane as reagent gas. The PFB derivatives of the 2-thiouracils gave mass spectra in which the ion current was carried largely by an ion arising from [M-PFB]-. The derivative was used in the determination of the uptake and metabolism of thiocarbamides by cultures of melanoma cells.

Topics: Gas Chromatography-Mass Spectrometry; Melanoma; Methimazole; Methylthiouracil; Propylthiouracil; Thiouracil; Tumor Cells, Cultured

2-Thiouracil (TU), an antithyroid drug, is receiving growing interest as a specific tumor marker for malignant melanoma, owing to its capability of being selectively accumulated into active melanin-producing tissues. However, up until now, the molecular mechanism of TU uptake by growing melanin has remained largely unknown. In an attempt to fill this gap, we have investigated the effect of TU on the tyrosinase catalyzed oxidation of tyrosine. At a concentration of 0.5 mM, TU was found to totally inhibit melanin formation by tyrosinase catalyzed oxidation of 0.25 mM tyrosine in phosphate buffer at pH 6.8. Polarographical monitoring of oxygen consumption under conditions of complete suppression of melanogenesis revealed a significant tyrosinase activity, with TU acting as a modest non-competitive inhibitor of the enzyme (Ki = 0.6 mM). HPLC and TLC analysis of the tyrosine-tyrosinase reaction in the presence of excess TU showed that the substrate is progressively consumed and a major hitherto unknown product (lambda max = 284 nm), positive to ninhydrin and ferric chloride, is concomitantly formed. This was isolated by repeated gel filtration chromatography of the reaction mixture on Sephadex G-10 and was formulated as the TU-dopa adduct 3,4-dihydroxy-6-(4'-hydroxypyrimidinyl-2'-thio)phenylalanine by spectral analysis. These results suggest that selective TU incorporation in pigmented melanomas and other melanin-producing systems is due to the covalent binding to dopaquinone, produced by tyrosinase catalyzed oxidation of tyrosine.

Topics: Animals; Benzoquinones; Biomarkers, Tumor; Chromatography, Gel; Chromatography, High Pressure Liquid; Chromatography, Thin Layer; Dihydroxyphenylalanine; Kinetics; Magnetic Resonance Spectroscopy; Melanins; Melanoma; Mice; Mice, Inbred C57BL; Monophenol Monooxygenase; Oxygen Consumption; Thiouracil; Tyrosine

5-Iodo-2-thiouracil (ITU) is of interest due to its ability to bind specifically to the pigment melanin during melanogenesis and is of potential value in the diagnosis and treatment of malignant melanoma. Radioiodinated ITU was prepared directly from 2-thiouracil in a two-phase reaction using Iodo-Gen in 0.05 M phosphate buffer pH 7.0. The identity radiochemical purity and stability of the product were checked by reversed-phase high pressure liquid chromatography (HPLC). ITU labeled with 123I, 125I or 131I has been produced in millicurie amounts and isolated on a semi-preparative reversed-phase HPLC column. Production time was 2-3 h, overall radiochemical yields averaged 80%; the radiochemical purity was greater than 98%. Specific activities on the order of 20 Ci/mmol have been obtained.

Topics: Animals; Chromatography, High Pressure Liquid; Humans; Iodine Radioisotopes; Melanins; Melanoma; Radiochemistry; Radionuclide Imaging; Thiouracil; Urea

Topics: Humans; Iodine Radioisotopes; Melanoma; Radionuclide Imaging; Thiouracil

The uptake (total radioactivity in intact cells) and incorporation (radioactivity bound to acid-precipitable material) of 14C-chlorpromazine (CPZ) and 14C-thiouracil (TU) were studied using a library of 3 human fibroblast strains and 13 tumour cell lines. In contrast to previous studies using rodent melanomas in vivo, the melanoma lines, including lines with high tyrosinase and melanin contents, did not take up more CPZ and TU than non-melanoma cells (fibroblasts, HeLa cells). Incorporation of CPZ was also broadly similar in all cell types studied. TU was selectively incorporated into the melanoma line having a high tyrosinase and melanin content but not into lines with high tyrosinase activity and low melanin content. While supporting the possibility of selective therapy for heavily-pigmented melanomas using labelled TU derivatives, these results suggest that the action of potentially melanoma-affined compounds should be further evaluated in human cells. Unlabelled CPZ or TU was not selectively toxic to melanoma cells. Unexpectedly, methylation-sensitive tumour cells (Mer-phenotype) were highly resistant to TU, thus providing a new experimental tool for understanding the genesis of this phenotype in vivo.

Topics: Cell Line; Chlorpromazine; Humans; Melanins; Melanoma; Monophenol Monooxygenase; Phenylthiourea; Thiouracil

Radio-iodine-labelled thiouracil has been evaluated as a radiopharmaceutical for establishing the viability of ocular melanoma after radiation treatment. The uptake of 125I-5-iodo-2-thiouracil (125I-ITU) was studied in X-ray irradiated and non-irradiated melanotic melanomas implanted in Syrian golden hamsters. Uptake of 125I-ITU in melanomas 4 days after irradiation with 40 Gy X-ray was 25% of the value found in non-irradiated controls, 12 days after such treatment it was 10% of that value. Twenty-one days after radiation treatment the melanomas showed regrowth and uptake of 125I-ITU was about equal to that in non-treated controls. Uptake of 125I-ITU in melanomas after 10 Gy X-ray irradiation was higher and uptake in tumours after 20 Gy was only slightly lower than the uptake by non-irradiated melanomas. The results indicate that the iodine labelled-thiouracil uptake test may be useful as an additional diagnostic issue for assessing the viability of ocular melanoma after radiation therapy.

Topics: Animals; Choroid Neoplasms; Cricetinae; Iodine Radioisotopes; Melanoma; Mesocricetus; Radionuclide Imaging; Thiouracil

[123I]-5-iodo-2-thiouracil (123I-ITU) was evaluated as a radiopharmaceutical for tumor detection in 10 patients with proven choroidal melanoma. Uptake of 123I-ITU was measured with a specially designed single eye probe collimator, 24 h after administration of 123I-ITU. Increased uptake in the tumor-bearing eye as compared to the fellow nontumor bearing eye was found in 7 out of 10 cases when the probe was located 3.5 cm in front of the eye (p less than 0.01). By using a double pinhole collimator tests were positive in 3 out of 10 123I-ITU studies only. Tests with 123I-ITU were compared with 67Ga tests in the same patients. The 67Ga tests with the single eye probe collimator were positive in 6 out 10 cases when the probe was located 6 cm in front of the eye. With the double pinhole collimator tests were positive in 7 out of 10 67Ga studies. It is concluded that 123I-labeled thiouracil is at least as useful as a radiopharmaceutical for ocular melanoma diagnosis as 67Ga-citrate, provided measurements are performed with a single eye probe.

Topics: Choroid Neoplasms; False Positive Reactions; Gallium Radioisotopes; Humans; Melanoma; Radionuclide Imaging; Thiouracil

Thiouracil and various derivatives are selectively incorporated into the melanin pigment of melanomas during biosynthesis by serving as false melanin precursors. Using the transplantable Harding-Passey melanoma carried in BALB/c mice, we have extended our previous studies with sulfur-35 (35S) thiouracil. The persistence of high levels of [35S]thiouracil in tumor for periods of up to 2 wk has been demonstrated; during this time the drug content in normal tissues returned to near background levels. The variety of iodine isotopes available makes iodothiouracil a particularly promising melanoma-localizing agent. Tumor uptake and biodistribution of [35S]thiouracil and iodothiouracil (both iodine-127 (127I) and iodine-125 (125I) labeled) have been compared and were found to be essentially the same. The selectivity of [125I]thiouracil for melanoma has been qualitatively demonstrated by autoradiography of whole-body sections and quantitated by analysis of tumor and selected tissues. Iodothiouracil was also shown to localize in remote secondary metastases using a metastatic variant of the Harding-Passey melanoma currently being developed in our laboratory. These studies confirm the melanoma localizing capabilities of an iodinated thiouracil, and therefore the potential of using iodinated thiouracil derivatives for diagnosis and therapy of melanotic melanomas.

Topics: Animals; Female; Iodine Radioisotopes; Melanoma; Mice; Mice, Inbred BALB C; Neoplasm Transplantation; Radionuclide Imaging; Sulfur Radioisotopes; Thiouracil; Tissue Distribution

Previous studies have demonstrated that retinoids possess antineoplastic properties against melanoma. The purpose of this study was to determine whether topically applied retinoic acid could prevent melanoma development in syngeneic mice after intracutaneous cell inoculation. Trans-retinoic acid in DMSO was applied daily for 28 days after melanoma implantation and tumor growth was quantitated by the uptake of [14C]thiouracil, a tracer compound specific for melanoma which is incorporated linearly according to the weight of the tumor. Marked reduction in tumor growth was noted at the highest concentration (0.1%) tested and lesser but significantly decreased tumor growth patterns were also realized at lower concentrations in a dose-dependent manner. Thus, topically applied retinoic acid is capable of inhibiting S91 melanoma growth in vivo.

Topics: Administration, Topical; Animals; Carbon Radioisotopes; Dimethyl Sulfoxide; Dose-Response Relationship, Drug; Male; Melanoma; Mice; Mice, Inbred DBA; Neoplasm Transplantation; Skin Neoplasms; Thiouracil; Tretinoin

The uptake of 123I-5-iodo-2-thiouracil in melanotic and amelanotic melanoma implanted in Syrian golden hamsters was studied. A selective accumulation was found in the tumours. Uptake of 123ITU in melanotic melanomas was 4 to 5 times the uptake in amelanotic ones. For both tumours high ratios of tumours versus non-tumour were found. The high accumulation of 123ITU in both kinds of tumours and the high tumour versus non-tumour ratios suggest that 123ITU may be a promising radiopharmaceutical for the detection of ocular melanoma.

Topics: Animals; Cricetinae; Eye Neoplasms; Iodine Radioisotopes; Male; Melanoma; Mesocricetus; Radionuclide Imaging; Thiouracil

The incorporation of [2-14C]-2-thiouracil and a series of [125I]-5-iodo-2-thiouracils ([125I]ISUra(s)) into cultured Greene hamster melanoma cells was determined in order to establish their properties as false precursors in the melanin-biosynthetic pathway. The cold trichloroacetic acid-precipitable incorporation of [2-14C]-2-thiouracil as well as [125I]ISUra into melanoma cells after a 24- to 48-hr labeling period proved to be completely tyrosinase dependent (more than 99.5% inhibition could be achieved by 0.5 mM phenylthiourea). [125I]ISUra incorporation was 3-fold higher than was [2-14C]-2-thiouracil incorporation and was enhanced by 1 mM theophylline treatment. [125I]ISUra incorporation into hamster, rabbit, and human melanoma cells showed a linear relationship with cell melanin content. Methylation of the sulfur completely prevented the incorporation, while propylation but not methylation at position 6 resulted in lower incorporation. [125I]ISUra proved to be a marker for melanogenesis and may be useful in studies on the differentiation of cultured melanoma cells.

Topics: Animals; Cell Division; Cells, Cultured; Cricetinae; Humans; Melanins; Melanoma; Phenylthiourea; Rabbits; Thiouracil

Thiouracil is a thiol-containing pyrimidine that is selectively incorporated into cells that synthesize melanin. In an effort to delineate further the specificity and dynamics of uptake, we injected thiouracil labeled with radioactive carbon into S91 melanoma-bearing mice; biopsy specimens were taken of the tumors and organs at various time intervals thereafter. The data showed a substantial uptake of thiouracil by the melanomas, with peak uptake occurring at 24 hours. All other organs examined showed only minor amounts of radioactivity, which probably reflected the presence of thiouracil in the blood perfusing these tissues. Because of its incorporation into melanomas, the use of radioactive thiouracil has potential as a marker for tumor growth, as a diagnostic tracer compound, and as a carrier for chemotherapeutic agents.

Topics: Animals; Carbon Radioisotopes; Cells, Cultured; Male; Melanins; Melanoma; Mice; Mice, Inbred DBA; Neoplasms, Experimental; Thiouracil; Tissue Distribution

The tissue distribution of a number of 5-[131I]-iodo-2-thiouracil derivatives was measured in Syrian golden hamsters with Greene melanoma. These compounds were rapidly (in less than 1 h) distributed in all tissues, while in most tissues fast elimination (T1/2 1-3 h) was observed. Because of retention of the 131I activity in the tumour, high tumour/non-tumour tissue ratios were found (e.g. tumour/eye 2.3-10, tumour/skin 1.5-3) suggesting that some of these compounds might be used as melanoma delineating agents, when labelled with 123I.

Topics: Animals; Cricetinae; Eye Neoplasms; Iodine Radioisotopes; Male; Melanoma; Mesocricetus; Neoplasm Transplantation; Neoplasms, Experimental; Radionuclide Imaging; Thiouracil; Tissue Distribution

The incorporation of 2-[35S]thiouracil and two of its derivatives into murine melanomas, in vivo, was studied. It was confirmed [J. R. Whittaker, J. Biol. Chem., 246, 6217--6226 (1971)] that 2-thiouracil has a marked affinity for melanin-producing tissue and that an affinity for such tissue could be sustained by 5-substituted 2-thiouracils. A series of derivatives of arotinoids and retinoids, with or without a 2-thiouracil group as a potential carrier to obtain affinity for melanomas, was examined for cytostatic activity, in vitro. None of these showed significant activity against murine melanomas.

Topics: Animals; Antineoplastic Agents; Cells, Cultured; Chemical Phenomena; Chemistry; Melanoma; Mice; Mice, Inbred DBA; Neoplasms, Experimental; Thiouracil; Tissue Distribution; Tretinoin

In a previous report we have shown that a few substances, especially thiouracil, are incorporated as false precursors into melanin during its synthesis. In the present investigation, we have intensified our studies on the incorporation of thiouracil into melanotic melanomas. Firstly, the distribution and retention of both 14C- and 35S-labelled thiouracil in mice with transplanted melanomas were studied. A high and selective accumulation was found in the melanotic tumours. The concentration in the rest of the body was low, with the exception of the thyroid gland. Secondly, melanoma-bearing mice were given increasing doses of thiouracil, and cultured melanoma cells were exposed to different concentrations of thiouracil, to investigate the relation between dose and uptake in melanomas and melanoma cells, respectively. A relatively linear increase in uptake with dose was found, indicating that the melanin incorporation of thiouracil is non-saturable up to subtoxic levels.

Topics: Animals; Autoradiography; Carbon Radioisotopes; Cells, Cultured; Dose-Response Relationship, Drug; Melanoma; Mice; Mice, Inbred Strains; Neoplasms, Experimental; Sulfur Radioisotopes; Thiouracil; Tissue Distribution

We have shown that thioamides are incorporated as false precursors into melanin during its synthesis. To be clinically useful in the diagnosis or therapy of melanotic melanomas, they would have to be tagged with an appropriate isotope or possibly a cytotoxic moiety. 125I-Thiouracil (125I-TU) is here shown to be accumulated in the melanin of melanotic melanomas transplanted into mice in a similar way as is 14C-thiouracil (14C-TU). 125I-TU gives tumour/liver and tumour/muscle ratios up to 22 and 778 respectively, at 4 days after administration. 125I-TU is accumulated by melanoma cells in vitro more effectively than 14C-TU (125I-TU/14C-TU, 2.7), while the in vivo accumulation into melanomas is slightly lower for 125I-TU as compared to 14C-TU (125I-TU/14C-TU, 0.35). This appears to be due to a partial deiodination (less than 14% of the dose within 4 days) and probably a more rapid excretion of 125I-TU or its metabolite(s). The accumulation of radioactivity in the thyroid can essentially be eliminated by pretreatment with potassium iodide and/or thyroxine. 125I-Propylthiouracil is also accumulated in melanotic melanoma cells in vivo and in vitro, but at a lower level than in 125I-TU and 14C-TU.

Topics: Animals; Cells, Cultured; Iodine Radioisotopes; Melanoma; Mice; Mice, Inbred DBA; Neoplasms, Experimental; Potassium Iodide; Propylthiouracil; Sodium Iodide; Thiouracil; Thyroxine; Tissue Distribution

Topics: Animals; Autoradiography; Bone Marrow; Carbon Radioisotopes; Intestine, Small; Kinetics; Melanoma; Mice; Mice, Inbred BALB C; Neoplasms, Experimental; Thiouracil; Tissue Distribution

Topics: Animals; Autoradiography; Eye Neoplasms; Melanoma; Mice; Neoplasm Transplantation; Neoplasms, Experimental; Radionuclide Imaging; Thiouracil; Tissue Distribution; Transplantation, Isogeneic

Topics: Animals; Cell Line; Cricetinae; Cycloheximide; Melanins; Melanoma; Neoplasms, Experimental; Stimulation, Chemical; Thiouracil; Tyrosine

Hamster melanoma cells (RPMI 3460) were examined for their ability to utilize phenylalanine for melanin biosynthesis. There was a small but significant incorporation of L-[1-1414C] phenylalanine into hot acid-insoluble cellular material in the presence of cycloheximide. However, this radioactivity was removable from the acid-insoluble fraction by pronase digestion. A similar percentage of L-[U-14C] leucine incorporation was likewise resistant to cycloheximide inhibition. Residual protein synthesis is apparently responsible for the incorporation of both amino acids. Cycloheximide did not inhibit melanin synthesis. These results suggest that mammalian melanocytes do not use phenylalanine for melanin synthesis. Phenylalanine is not incorporated directly into melanin, nor do the cells appear to convert it to tyrosine via a phenylalanine hydroxylase.

Topics: Animals; Cell Line; Cricetinae; Cycloheximide; Leucine; Melanins; Melanocytes; Melanoma; Phenylalanine; Pronase; Thiouracil; Tyrosine

Topics: Animals; Fundulidae; Growth and Development; Melanoma; Melanoma, Cutaneous Malignant; Melanoma, Experimental; Skin Neoplasms; Thiouracil; Thyroxine