The chemistry and evaluation of porphyrin-based potential anti-cancer agents
Creators: name: Posakony, Jeffrey Jerard
Publisher: name: University of British Columbia
Year: 2009

description: Towards the goal of synthesizing porphyrin-based radiosensitizers and hypoxia-selective cytotoxins (HSCs) leading to improved cancer treatment modalities, the chemistry of several porphyrin systems (diarylporphyrins, tetraarylporphyrins, and those based on protoporphyrin IX) was investigated. Porphyrin incorporation of nitroaromatic and heterocyclic-N-oxide moieties into porphyrins was the general objective, but additional areas were investigated. [chemical compound diagrams] The porphyrin precursor bis(2-pyrrolyl)methane (3) was obtained (84 %) by NaBH₄- or (80 %) LiAlH₄- reduction of bis(2-pyrrolyl)thioketone (1), a substantial improvement over current methodologies. Raney Ni-reduction of 1 (~50 %) also produced the novel l,l',2,2'-tetrakis(2- pyrrolyl)ethane (4). Using 3 and an appropriate aldehyde, symmetrical diarylporphyrins incorporating pyridyl, oxidppyridyl-, nitrophenyl and phenyl substituents were synthesized. The highest yield was obtained for 5,15-diphenylporphyrin (25-40 %) which was subsequently mesobrominated and metallated (Zn); this complex was subsequently used in Stille-type Pd-catalyzed cross-coupling reaction attempts using a variety of organotin reagents. The reactivity of one such cross-coupling reaction product, [5,15-diphenyl-10,20-divinylporphyrinato]Zn(II) (DPhDVPZn, 77 %), reported previously, was explored via chemistry at the vinyl group, but most conditions attempted gave product mixtures in low yield. Reaction of DPhDVPZn with Tl III led to rapid porphyrin decomposition, but the demetallated vinyl analog, DPhDVP, reacted cleanly to afford 5,15-bis(2,2-dimethoxyethyl)-10,20-diphenylporphyrin (69 %). Condensation of pyrrole with one or two aldehydes in propionic acid (the Adler method) was used to synthesize several tetraarylporphyrins containing pyridyl or imidazolyl groups. The phenyl-pyridyl series was easiest to work with, while porphyrins containing imidazolyl groups were produced in low yield and were difficult to purify. Attempts to form porphyrins from heterocyclic, aromatic aldehydes via Lindsey conditions were generally unsuccessful and a possible explanation is presented. Nitration of meso-tetrakis(2-imidazolyl)porphyrin using several conditions was unsuccessful. Porphyrins containing one to four pyridyl groups were 'N-oxidized' with m-chloroperbenzoic acid to produce five novel (oxidopyridyl)porphyrins and seven porphyrin Noxides. Sulfonation of the phenyl substituents 5-(1-oxido-4-pyridyl)- 10,15,20-triphenylporphyrin and cis-5,10-bis(1-oxido-4-pyridyl)-10,20-diphenylporphyrin yielded their water-soluble derivatives, OPyTrSPhP and c-BOPyBSPhP, respectively. Some metallation reactions with Pt were pursued; tetrakis(4-sulfonatophenyl)porphyrin was readily metallated using K₂PtCl₄, but similar conditions with pyridyl-tris(4-sulfonatophenyl)porphyrin led to porphyrin decompositic OPyTrSPhP was metallated with K₂PtCl₄, but the resulting product evidently had an externally bound Pt moiety, perhaps ligated through the oxidopyridyl group. [chemical compound diagrams] The product from tirapazamine (3-amino-1,2,4-benzotriazine-1,4-di-N-oxide) and triphosgene, 21, reacts like an isocyanate, and was used in reactions with carboxylic acids, alcohols, and amines to produce compounds (some new) incorporating tirapazamine. A tirapazamine-porphyrin conjugate (TirapPhTrPhP) was obtained from 21 and 5-(4-aminophenyl)- 10,15,20-triphenylporphyrin. New substituents were introduced at the 8,13-positions of protoporphyrin IX dimethylester via the previously reported Tl III-vinyl oxidation product, and a selective deprotection of this compound's dimethyl acetal functionalities was developed. The deprotected product, dimethyl 3,7,12,17-tetramethyl-8,13-bis(2-oxoethyl)-porphyrin-2,18-dipropionate (BOEtPIXDME), was subsequently converted to derivatives incorporating aniline and C₂F₅CH₂NH- groups via reductive amination chemistry; a -COOH substituted vinyl group was incorporated via a Knoevenagel reaction. Other reductive amination reactions with en, tirapazamine and NH₄OAc met with limited success. Aspects of the successful reductive amination reactions are discussed. [chemical compound diagrams] The aldehyde moieties of BOEtPIXDME were reduced using NaBH₄ and the resulting bis(2-hydroxyethyl) product (BHEtPIXDME) was used to synthesize the 8,13-tosyl-, iodo- and bromoethyl derivatives. Novel porphyrins incorporating two 2-nitroimidazolyl (BNImEtPIXDME) or two phthalimido moieties (BPlEtPIXDME) were obtained in subsequent SN₂ displacement reactions using the 8,13-bis(2-bromoethyl) derivative; elimination side-products were also observed (e.g. 29). Attempts to cleave the phthalimido groups of BPlEtPIXDME met with limited success. Acid-hydrolysis of BHEtPIXDME and BNImEtPIXDME yielded the carboxylic acid derivatives, BHEtPIX and BNImEtPIX. The solubility of BNImEtPLX was improved by formation of the 2,18-bis(L-aspartyl) amide. Attempts to incorporate other 2- nitroimidazoles and tirapazamine into derivatives of protoporphyrin IX were unsuccessful. Selected porphyrins were evaluated by cyclic voltammetry in DMF. Based on reported E1/2 and so called E₁⁷ values, oxidopyridyl, nitrophenyl, tirapazamine, and 2-nitroimidazolyl substituent and porphyrin ring reduction potentials are assigned. The usefulness of porphyrins containing these substituents as radiosensitizers and HSCs is discussed by comparing these reduction potentials to those of known radiosensitizers and HSCs. In vitro assays for radiosensitization, hypoxia-selective toxicity, and photosensitization with Chinese hamster ovary cells were used to evaluate the potential of selected porphyrins incorporating oxidopyridyl (OPyTrSPhP), 2-nitroimidazolyl (BNImEtPIX) or tirapazamine (TirapPhTrPhP) substituents along with suitable 'reference' compounds. The cellular accumulation of these porphyrins was evaluated by UV-Vis spectroscopy or fluorescence microscopy. Because of the hydrophobic nature of some of these compounds, liposomeformulations were developed for three of the porphyrins whereas the others were evaluated as solutions in a-modified medium. Cremophor EL® emulsions of the hydrophobic porphyrins were also tested. The results of the assays are compared with available literature data. In general, the porphyrins were non-toxic, and they showed little radiosensitizing or photosensitizing ability; however, TirapPhTrPhP showed a modest radiation SER value (1.5). Some photosensitization was observed with OPyTrSPhP, but its effectiveness was poor in comparison to that of Photofrin II®; some evidence was found for protection from PDT-induced damaged by pyridine N-oxide. Based on the accumulation in cells measured by UV-Vis spectroscopy, BNImEtPIX was accumulated to the greatest degree, but, per microgram of porphyrin delivered, Photofrin II® was accumulated the most and the sulfonatophenyl porphyrins the least. The accumulation data for the liposome-formulated porphyrins obtained via UV-Vis measurements appear to conflict with those from the fluorescence microscopy; some possible explanations are discussed.