Targeting Melanoma

via the Reactivity of Melanin

     We are investigating new chemotherapy strategies for melanoma cancer, based on the unique chemistry of the pigment melanin. Melanin is a broad class of highly colored compounds that give your skin and hair its color. The pigments are somewhat reactive, we call them pro-oxidants because they react with oxygen in the air to form toxic species like peroxide. The special cells that make these pigments, called melanocytes, tightly control the way these pigments are generated and distributed (like when your skin tans). In cancerous melanoma cells, the regulation of these pigments is disrupted and they seem more reactive. We are trying to develop drugs that utilize this difference in pigment reactivity between normal melanocytes
and melanoma cells.

    Melanins are polymeric pigments formed from the sequential oxidation of tyrosine. Two general types are known, black eumelanins due to the polymerization of dihydroxyindole precursors and red/brown pheomelanins, which are due to cysteine incorporation during the oxidations. In melanocytes, a peroxide-dependent enzyme, tyrosinase, catalyzes two successive reactions, the hydroxylation of tyrosine and the oxidation of the product, L-dopa. The product of dopa oxidation cyclizes to a 5,6-dihydroxyindole (DHI) intermediate, which is highly reactive and gives rise to eumelanin polymers by a pathway dependent on further oxidation by peroxidases and oxygen. Pheomelanins are thought to be engendered by the interception of dopaquinone with cysteine, which results in the incorporation of a benzothiazine monomer into the polymer.

   Although often presented as separate forms, eumelanin and pheomelanin are rather qualitative descriptions of a wide variety of native melanins, likely co-polymers with both indolic (eu-) or benzothiazine (pheo-) subunits. A number of chemical analysis of biological samples have shown that the largest component of natural melanins are eumelanic DHI subunits even in red and blond subjects. We focus on the chemical reactivities assignable to the dihydroxyindole (DHI) subunits, and on chemical transformations that occur under oxidative stress.

 

  Using electrochemically-generated synthetic melanins, we have investigated the speciation, or redox equilibrium, within melanins. Our initial work identified an unusual chemical species, the quinone-imine or QI, whose concentration tracks with metal-binding and chemical degradation. The QI has a very high affinity for metal, Ka > 10¹¹ for Cu(II) at pH 7.  As a major consequence, melanin's metal-binding ability increases upon partial oxidation, but beyond a certain point oxidation becomes chemically irreversible and melanin degradation occurs.

 

      

  The apparent QI-content tracks with a melanin's susceptibility to auto-oxidation, i.e. redox-cycling in the presence of oxygen to form reactive oxygen species (ROS) such as superoxide and peroxide. Using the radical trap DMPO and other methods, we have examined melanin's susceptibility to produce ROS during redox-cycling. The rate of formation of ROS by melanin/oxygen can be increased by either oxidative stress or metal-binding. Following similar procedures, we have measured ROS formation engendered by melanoma cells under oxidative and metal-based stress. We have also shown that these cancer cells are much more sensitive to oxygen itself than are normal melanocytes.

 

    We believe that the loss of structure of the melanin within melanosomes of melanoma make these cancers susceptible to chemotherapeutic targeting. It is significant that increased oxygen alone selectively targets melanoma in culture, and is partial proof of our hypothesis that these cancers are uniquely susceptible to oxidative stress. Our chemical model systems demonstrate that oxidized melanins have increased metal-binding and a higher reactivity with oxygen; thus a transformation to a pro-oxidant behavior may be induced by metal/oxygen combinations.

    Metal-based drugs for melanoma

   Initial cytotoxicity studies show that lipophilic metal-dithiocarbamate complexes have high anti-melanoma activity, and that this toxicity increases with increased partial pressure of oxygen. The unusual ability of dithiocarbamates to induce metal-uptake and apoptosis was first shown by Nobel in 1995, in the last decade many labs have shown this phenomena in various cell lines.

   In contrast to most such previous studies, we utilize the metal complexes themselves rather than mixtures of free chelate and metal salts. We have also synthesized several new series of metal complexes which have strong anti-melanoma activity. Ongoing structure/function analysis, as well as efforts to identify the intracellular target of these drugs, are carried out through collaboration with the Meyskens' lab at the Chao Family Cancer Center at UC Irvine. Our investigations have led to several new ways of modifying and derivatizing dithiocarbamate complexes and related sulfur-containing chelates, which we believe will open new avenues for their medical applications.

   Selected recent publications on melanin and melanoma:

“Targeting Melanoma via Metal-based Stress” Farmer, P.J.; Brayton, D.; Moore, C.; Williams, D.; Shahandeh, B.; Cen, D.; Meyskens, F.L. 2004, in press as invited chapter, ACS Monograph Series on Medicinal Inorganic Chemistry.

“The Effect of Stacking and Redox State on Optical Absorption Spectra of Melanins- a comparison of theoretical and experimental results.” Stark, K.B.; Gallas, J.M.; Zajac G.W.;  Golab, J.T.; Gidanian, S.; McIntire, T.; Farmer, P.J. submitted J. Phys. Chem.

“Etiologic Pathogenesis of Melanoma: A Unifying Hypothesis for the Missing Attributable Risk” Meyskens, F.L.; Farmer, P.J.; Anton-Culver, H. Clin. Canc.  Res.  2004, 10, 2581-2583.

"Redox Behavior of Melanins: Direct Electrochemistry of DHI-melanin and its Cu and Zn Adducts " Gidanian, S.; Farmer, P.J. J. Inorg. Biochem. 2002, 89, 54-60.

"Metal Binding by Melanins: Studies of Colloidal DHI-Melanin, and its Complexation by Cu(II), and Zn(II) Ions " Szpoganicz, B.; Gidanian, S.;  Kong, P.; Farmer, P.J J. Inorg. Biochem. 2002, 89, 45-53.

“Redox Regulation in Human Melanocytes and Melanoma” Meyskens, F.L.; Farmer, P.J.; Fruehauf, J. Pigment Cell Res., 2001, 14, 148-154.

   Selected recent publications on dithiocarbamates:

“Partial Sulfur Oxygenation of  bis-Diethyldithiocarbamate Zinc(II) Increases Apoptotic Response in Human Melanoma Cells” Brayton, D.; Midyett, J.; Khiterer, M.; Kolahi, K.; Meyskens, F.L.; Farmer, P.J. in preparation.

“Multiple Pathways for the Oxygenation of a Ru(II) Dithiocarbamate Complex: S-Oxygenation and S-Extrusion” Ng, S.;  Ziller, J.;  Farmer, P.J. in press Inorg. Chem.

“Disulfiram Causes Intracellular Cu Uptake and Induces Apoptosis in Human Melanoma Cells” Cen, D.; Brayton, D.; Shahandeh, B.; Meyskens, F.L.; Farmer, P.J.; in press J. Med. Chem.

“Melanin as a target for melanoma chemotherapy: pro-oxidant effect of oxygen and metals on melanoma viability.” Farmer, P.J.; Gidanian, S.; Shahandeh, B.; Di Bilio, A.J.; Tohidian, N.; Meyskens, F.L. Pigment Cell. Res. 2003, 16, 273-279.