Si(IV)-naphthalocyanine: modulation of its pharmacokinetic properties through the use of hydrophilic axial ligands

Optimizing Axial and Peripheral Substitutions in Si-Centered Naphthalocyanine Dyes for Enhancing Aqueous Solubility and Photoacoustic Signal Intensity

Photoacoustic imaging using external contrast agents is emerging as a powerful modality for real-time molecular imaging of deep-seated tumors. There are several chromophores, such as indocyanine green and IRDye800, that can potentially be used for photoacoustic imaging; however, their use is limited due to several drawbacks, particularly photostability. There is, therefore, an urgent need to design agents to enhance contrast in photoacoustic imaging. Naphthalocyanine dyes have been demonstrated for their use as photoacoustic contrast agents; however, their low solubility in aqueous solvents and high aggregation propensity limit their application. In this study, we report the synthesis and characterization of silicon-centered naphthalocyanine dyes with high aqueous solubility and near infra-red (NIR) absorption in the range of 850-920 nm which make them ideal candidates for photoacoustic imaging. A series of Silicon-centered naphthalocyanine dyes were developed with varying axial and peripheral substitutions, all in an attempt to enhance their aqueous solubility and improve photophysical properties. We demonstrate that axial incorporation of charged ammonium mesylate group enhances water solubility. Moreover, the incorporation of peripheral 2-methoxyethoxy groups at the α-position modulates the electronic properties by altering the π-electron delocalization and enhancing photoacoustic signal amplitude. In addition, all the dyes were synthesized to incorporate an N-hydroxysuccinimidyl group to enable further bioconjugation. In summary, we report the synthesis of water-soluble silicon-centered naphthalocyanine dyes with a high photoacoustic signal amplitude that can potentially be used as contrast agents for molecular photoacoustic imaging.

Axial PEGylation of Tin Octabutoxy Naphthalocyanine Extends Blood Circulation for Photoacoustic Vascular Imaging

Attachment of polyethylene glycol (PEG) can prolong blood circulation of biological molecules, a useful trait for a vascular imaging agent. Here, we present a route for modifying octabutoxy naphthalocyanine (ONc) with PEG, via axial conjugation following ONc chelation with Sn(IV) chloride (Sn-ONc). Tin chelation caused ONc absorbance to shift from 860 to 930 nm. Hydroxy terminated PEG was treated with sodium and then was axially attached to the tin, generating PEG-Sn-ONc. Unlike ONc or Sn-ONc, PEG-Sn-ONc was soluble in methanol. ONc and PEG-Sn-ONc were dissolved in polysorbate solutions and administered to mice intravenously. PEG-Sn-ONc demonstrated substantially longer blood circulation time than ONc, with a 4 times longer half-life and a nearly 10 times greater area under the curve. PEG-Sn-ONc gave rise to photoacoustic contrast and could be used for noninvasive brain vessel imaging even 24 h following injection. This work demonstrates that nonmetallic naphthalocyanines can be chelated with tin, and be axially modified with PEG for enhanced circulation times for long-term vascular imaging with photoacoustic tomography.

State of the art in the delivery of photosensitizers for photodynamic therapy

In photodynamic therapy, one of the problems limiting the use of many photosensitizers (PS) is the difficulty in preparing pharmaceutical formulations that enable their parenteral administration. Due to their low water solubility, the hydrophobic PS cannot be simply injected intravenously. Different strategies, including polymer-PS conjugation or encapsulation of the drug in colloidal carriers such as oil-dispersions, liposomes and polymeric particles, have been investigated. Although these colloidal carriers tend to accumulate selectively in tumour tissues, they are rapidly taken up by the mononuclear phagocytic system. In order to reduce this undesirable uptake by phagocytic cells, long-circulating carriers that consist of surface modified carriers have been developed. Moreover, considerable effort has been directed towards using other types of carriers to improve tumour targeting and to minimize the side effects. One of the approaches is to entrap PS into the lipophilic core of low-density lipoproteins (LDL) without altering their biological properties. The LDL receptor pathway is an important factor in the selective accumulation of PS in tumour tissue owing to the increased number of LDL receptors on the proliferating cell surface. Specific targeting can also be achieved by binding of monoclonal antibodies or specific tumour-seeking molecules to PS or by the coating of PS loaded carriers.

Water-soluble aluminium phthalocyanine-polymer conjugates for PDT: photodynamic activities and pharmacokinetics in tumour-bearing mice

The potential use of unsubstituted aluminium phthalocyanine (AlClPc) as a sensitizer for photodynamic therapy (PDT) of cancer has not been fully exploited in spite of its higher efficiency as compared to the sulphonated derivatives. This is largely due to the strong hydrophobic character of AlClPc which renders the material difficult to formulate for in vivo administration. We prepared two water-soluble derivatives of AlClPc by axial coordination of polyethyleneglycol (PEG, MW 2000) or polyvinylalcohol (PVA, MW 13,000-23,000) to the central aluminium ion. Their photodynamic activities were evaluated in vitro against the EMT-6 mouse mammary tumour cells and in vivo against the EMT-6 and the colon carcinoma Colo-26 tumours implanted intradermally in Balb/c mice. Pharmacokinetics were studied in the EMT-6 tumour-bearing mice. After 1 h incubation, the light dose required to kill 90% of cells (LD90) was at least three times less for AlClPc (Cremophor emulsion) as compared to AlPc-PEG and AlPc-PVA, while after 24 h incubation all three preparations were highly phototoxic. All three dye preparations induced complete EMT-6 tumour regression in 75-100% of animals at a low drug dose (0.25 micromol kg(-1)) following PDT (400 J cm(-2), 650-700 nm) at 24 h pi. Complete tumour regression in the Colo-26 tumour model was obtained in 30% of mice at a dose of 2 micromol kg(-1). In the non-cured animals, AlPc-PVA induced the most significant tumour growth delay. This dye showed a prolonged plasma half-life (6.8 h) as compared to AlClPc (2.6 h) and AlPc-PEG (23 min), lower retention by liver and spleen and higher tumour-to-skin and tumour-to-muscle ratios. Our data demonstrate that addition of hydrophilic axial ligands to AlPc, while modifying in vitro and in vivo kinetics, does not reduce the PDT efficiency of the parent molecule. Moreover, in the case of the polyvinylalcohol derivative, axial coordination confers advantageous pharmacokinetics to AlPc, which makes this photosensitizer a valuable, water soluble candidate drug for clinical PDT of cancer.

Substituted tetra-2,3-pyrazinoporphyrazines. Part II. Bis(tri-n-hexylsiloxy)silicon derivatives

Dichlorosilicon complexes of substituted tetra-2,3-pyrazinoporphyrazines were obtained via condensation of 2,3-dicyanopyrazine, 2,3-dicyano-5,6-diphenylpyrazine, 2,3-dicyanoquinoxaline, 2,3-dicyano-benzo[f]quinoxaline, and 2,3-dicyano-dibenzo[f,h]quinoxaline with silicon tetrachloride in the presence of urea, quinoline, and tri-n-butylamine. Hydrolysis of the Si-Cl bond in concentrated H2SO4, followed by treatment with 0.01 N NaOH and aqueous NH3, afforded the corresponding dihydroxides, which were converted to the bis(tri-n-hexylsiloxy)silicon derivatives via reaction with tri(n-hexyl)silane in 3-picoline (2,4,6-collidine) in the presence of tri-n-butylamine. The axial tri-n-hexylsiloxy substituents at the central silicon atom prevent aggregation in organic solvents, permitting detailed studies on the effects of structural modifications on the electronic spectra of tetraazaphthalocyanines. Our data show that each benzo ring addition, angularly condensed to the tetra-2,3-quinoxalinoporphyrazine, induces a hypsochromic shift (~ 10–15 nm) of the main absorption maximum. Key words: phthalocyanine, aza analog, bis(tri-n-hexylsiloxy)silicon complex.

Effect of axial ligation and delivery system on the tumour-localising and -photosensitising properties of Ge(IV)-octabutoxy-phthalocyanines

Four Ge(IV)-octabutoxy-phthalocyanines (GePcs) bearing two alkyl-type axial ligands were assayed for their pharmacokinetic properties and phototherapeutic efficiency in Balb/c mice bearing an intramuscularly transplanted MS-2 fibrosarcoma. The GePcs were i.v. injected at a dose of 0.35 mumol kg-1 body weight after incorporation into either Cremophor emulsions or small unilamellar liposomes of dipalmitoyl-phosphatidylcholine (DPPC). Both the nature of the delivery system and the chemical structure of the phthalocyanine were found to affect the behaviour of the GePcs in vivo. Thus, Cremophor-administered GePcs invariably yielded a more prolonged serum retention and a larger association with low-density lipoproteins (LDLs) as compared with the corresponding liposome-delivered phthalocyanines. This led to a greater efficiency and selectivity of tumour targeting. These effects were more pronounced for those GePcs having relatively long alkyl chains (hexyl to decyl) in the axial ligands. Maximal tumour accumulation (0.67 nmol per g of tissue) was found for Ge-Pc(hexyl)2 at 24 h after injection. Consistently, the Ge-Pc(hexyl)2, administered via Cremophor, showed the highest phototherapeutic activity towards MS-2 fibrosarcoma.

CGP 55398, a liposomal Ge(IV) phthalocyanine bearing two axially ligated cholesterol moieties: a new potential agent for photodynamic therapy of tumours

Ge(IV) phthalocyanine (GePc) with two axially ligated cholesterol moieties was prepared by chemical synthesis and incorporated in a monomeric state into small unilamellar liposomes (CGP 55398). Upon photoexcitation with light wavelengths around its intense absorption peak at 680 nm, GePc shows an efficient photosensitising activity towards biological substrates through a mechanism which largely involves the intermediacy of singlet oxygen. GePc injected systemically into mice bearing an intramuscularly implanted MS-2 fibrosarcoma is quantitatively transferred to serum lipoproteins and localises in the tumour tissue with good efficiency: at 24 h post injection the GePc content in the tumour is 0.74 and 1.87 micrograms per g of tissue with a tumour/peritumoral ratio of 4.35 and 5.67 for injected doses of 0.76 and 1.52 mg kg-1 respectively. At this time the red-light irradiation of the GePc-loaded fibrosarcoma causes a fast and massive tumour necrosis involving both malignant cells and blood vessels.

Hydrophobic Zn(II)-naphthalocyanines as photodynamic therapy agents for Lewis lung carcinoma

Four Zn(II) 2,3-naphthalocyanines (unsubstituted ZnNc1, tetracetylamido substituted ZnNc2, tetramino substituted ZnNc3 and tetramethoxy substituted ZnNc4) incorporated into unilamellar liposomes of dipalmitoylphosphatidylcholine have been injected intra-peritoneally (i.p.) (0.25-0.3 mg kg-1) to male C57/Black mice bearing a transplanted Lewis lung carcinoma. The pharmacokinetic investigations show that three of the four studied ZnNcs, 1, 2 and 4, are good tumor-localizers in Lewis lung carcinoma. The highest concentration is detected after ZnNc1 administration. The lowest tumor concentration as well as the lowest phototherapeutic effect were established with ZnNc3. In previous work it was shown that this ZnNc did not differ from the other three studied ZnNcs regarding the quantum yield of 1O2-formation and the photoinduced electron transfer. Obviously not only the good photochemical properties but also the tumor drug uptake can be an important factor of effective PDT. The biodistribution investigations also show that 72 h after drug injection, the skin concentration of the studied ZnNcs returns to the original base line. Indeed, we can expect that the skin photosensitivity will last for no longer than three days after PDT. The established higher drug concentration in the tumor rather than in the liver tissue (20 h after injection) shows again the tumor targeting selectivity of the applied liposome-sensitiser delivered procedure. Evaluating the PDT effect as reflected in the dynamics of the mean tumor diameter, we obtained unambiguous data on the potential capacity of ZnNcs 1,2,4 as PDT-photosensitisers. The data obtained from the assessment of the cytotoxic effect of PDT on the basis of the degree of induced necrosis, gave an adequate characterization of the tumor tissue destruction.(ABSTRACT TRUNCATED AT 250 WORDS)

The role of lipoproteins in the delivery of tumour-targeting photosensitizers

1. Serum lipoproteins play an important role in the in vivo transport of several porphyrinoid derivatives having a moderate or high degree of hydrophobicity. 2. There appears to exist a correlation between the extent of photosensitizer association with low-density lipoproteins (LDL) and the efficiency of tumour targeting by some classes of photosensitizers, such as differently sulphonated porphyrins and phthalocyanines, haematoporphyrin dialkylethers and unsubstituted phthalocyanines and naphthalocyanines. 3. In all cases, LDL-carried photosensitizers are preferentially released to malignant cells; hence, direct cell damage appears to be the major determinant of tumour damage consequent to photodynamic therapy. 4. Present evidence suggests that the LDL-associated photosensitizer is accumulated by tumour cells largely via a receptor-mediated endocytotic process. 5. Thus, the use of delivery systems for orientating a systemically injected photosensitizer towards lipoproteins has been explored; promising results have been obtained by incorporation of the dye into liposomal vesicles, oil emulsions or inclusion complexes, as well as by precomplexation of the dye with LDL. 6. Moreover, a suitable choice of the chemical constituents of the delivery system and the experimental conditions allows one to modulate the photosensitizer distribution among the different lipoproteins. 7. The occurrence of tumour-targeting strategies other than the LDL pathway is briefly discussed.

Binding of drugs to human plasma proteins, exemplified by Sn(IV)-etiopurpurin dichloride delivered in cremophor and DMSO

1. The mode-delivery-effect upon the binding of Sn(IV)-etiopurpurin dichloride (SnET2) in human plasma has been studied by ultracentrifugation, combined with absorption and fluorescence spectroscopy. SnET2 was delivered to plasma either in Cremophore EL (CRM) or in dimethyl sulfoxide (DMSO). To facilitate interpretation, optical, conductivity and aggregation properties of SnET2 were obtained for various solutions. 2. The second order rate constant for the aggregation of SnET2 monomers seemed to be remarkably small, of the order of 10(3) M-1 min-1. 3. SnET2 was bound as monomeric entities. Such entities had environmental-sensitive fluorescent properties dependent on the type of protein or solvent (DMSO, CRM, H2O) with which they interacted. 4. SnET2 showed saturable binding with high density subfraction(s) of high density lipoproteins and with one or more high density proteins. Complete or substantial saturation was achieved at the SnET2 level of 3.5 micrograms/ml. Such binding might be mediated by apolipoprotein D and alpha 1-acid glycoprotein. 5. There was little effect of SnET2 concentrations (3.5-35 micrograms SnET2/ml) upon the plasma binding of SnET2, irrespective of the mode of delivery. 6. The percentages of SnET2 bound to low density lipoproteins (LDL), high density lipoproteins (HDL), and high density proteins (HDP) were 10, 70 and 20%, respectively, for delivery in DMSO. The value for LDL also includes binding with very low density lipoproteins (VLDL). For delivery in CRM the corresponding values were 20, 50 and 30%. Apparently, CRM interacted with HDL entities and reduced their affinity for SnET2. 7. The distribution pattern of SnET2 among lipoproteins reflects interactions with apoproteins and/or with surface phospholipids rather than with core lipid constituents of lipoproteins. 8. Conductivity measurements showed that SnET2 was partly an ionic entity in water. 9. The plasma binding of SnET2 is compared with the corresponding binding of other drugs, both tetrapyrroles and nontetrapyrroles.