Therapeutic Potential of Tin Compounds Without Tinto Carbon Bonds

While the emphasis of this review thus far focussed upon organotin compounds, owing to the large number of ongoing investigations seeking therapeutic potential of this class of compound, non-organotin compounds also play a role in medicine. In the same way, only Sn(IV) compounds have been mentioned but in the realm of 'inorganic' tin, both Sn(II) and Sn(IV) feature. Indeed, SnF2 has been utilized in dental hygiene to inhibit dental plaque formation owing to its apparent prophylactic and therapeutic character.7,8 Local destruction on a tooth arises from decalcification of the tooth enamel and may, after enzymatic lysis, lead to the formation of a cavity. SnF2 is incorporated in less than 1% fraction in tooth-paste marketed as, for example, Crest® and Gel Tin®.

Early work showed that Sn(IV) porphyrin complexes may be used clinically in the treatment of jaundice (icterus), a physiological condition found in newborn babies.8 Neo-natal jaundice, hyperbilirubinaemia, occurs when the liver is not able to detoxify bilirubin, a bile pigment, and results in the yellow staining of tissue. The complex dichlorotin(IV) protoporphyrin IX (Figure 22.6a) is an inhibitor of haemoxidase, an enzyme responsible for the degradation, in the presence of O2 and reduced Nicotinamide Adenine Dinucleotide Phosphate (NADPH), of haemoglobin to Fe(II) ions, CO and biliverdin; biliverdin is reduced to bilirubin by biliversin reductase. This control of haeme metabolism has an implication for human immunodeficiency virus (HIV) treatment. The observation that porphyrins can bind to the V3 loop of the envelope glyco-protein on HIV type 1 (HIV-1) and inhibit infection of cells by HIV-1 prompted a study of the anti-HIV-1 activity of dichlorotin(IV) protoporphyrin IX.104 The conclusion of the study, which showed that the Sn(IV) complex binds and distorts the native conformation of the HIV-1 envelope and inhibits infection,

Figure 22.6 Chemical structures of (a) dichlorotin(IV) protoporphyrin IX, and (b) tin ethyl etiopurpurin was that dichlorotin(IV) protoporphyrin IX could also be used as a prophylactic and therapeutic against HIV-1.104 In a related study, using the principles of PhotoDynamic Therapy (PDT), dichlorotin(IV) protoporphyrin IX was evaluated for the treatment of Kaposi's sarcoma, a highly visible tumour that is frequently seen in patients with HIV.105 The above led to two vitally important events in therapeutic tin pharmacology. Firstly, a patent has recently appeared with the title 'Methods for Preventing HIV-1 or HIV-2 Infection' in which it is claimed that dichlorotin(IV) protoporphyrin IX, amongst others, is pharmaceutically effective against HIV-1 and HIV-2.106 Secondly, other tin-based porphyrins were investigated for utility in PDT.

PhotoDynamic Theory is a relatively new procedure that can be employed in the treatment of skin cancers which comprise approximately one-third of all cancers suffered by humans. Examples of skin cancers are (i) the aforementioned Kaposi's sarcoma, (ii) solar keratoses, afflicting the face and scalp and (iii) basal cell carcinomas, one of the most common forms of skin cancer and one correlated with chronic exposure to sunlight. The basic principle of PDT is that photosensitizing agents, light and oxygen are employed in a chemical reaction localized at the cancer and designed to selectively kill cancer cells once light of a specific wavelength interacts with the photosensitizing agent. Tin ethyl etiopurpurin (Figure 22.6b) has attracted significant attention in this regard.107 110 Tin ethyl etiopurpurin has also been investigated for other therapeutic potential.

Very recently, Miravant Medical Technologies,111 a company specializing in 'photoselective medicine', has announced that the US Food and Drug Administration has accepted a New Drug Application submitted by Miravant. Tin ethyl etiopurpurin is being marketed for the treatment of wet age-related macular degeneration (AMD). AMD is associated with a loss of central vision owing to the leaking of various fluids, including blood, at the back of the eye by abnormal blood vessels; indeed, AMD is a leading cause of blindness in older adults. The drug is administered using the principles outlined above for PDT.

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