Albumin-Folate Conjugates for Drug-targeting in Photodynamic Therapy

Distribution of photoactive β-carboline alkaloids across Passiflora caerulea floral organs

This study reports valuable information regarding the presence and concentration of a series of photoactive β-carboline (βCs) alkaloids (norharmane, harmane, harmine, harmol, harmaline, and harmalol) and their distribution across the floral age and organs of Passiflora caerulea. UHPLC-MS/MS data reported herein reveal that the βCs' content ranged from 1 to 110 μg kg-1 , depending on the floral organ and age. In certain physiologically relevant organs, such as anthers, βCs' content was one order of magnitude higher than in other organs, suggesting a special role for βCs in this specific organ. βCs' content also varied in a structure-dependent manner. Alkaloids bearing a hydroxyl group at position C(7) of the main βC ring were present at concentrations one order of magnitude higher than other βC derivatives investigated. UV-visible and fluorescence spectroscopy of the flower extracts provided complementary information regarding other biologically relevant groups of chromophores (phenolic/indolic derivatives, flavonoids/carotenes, and chlorophylls). Since flowers are constantly exposed to solar radiation, the presence of photoactive βCs in floral organs may have several (photo)biological implications that are further discussed.

Carrying Temoporfin with Human Serum Albumin: A New Perspective for Photodynamic Application in Head and Neck Cancer

Temoporfin (mTHPC) is approved in Europe for the photodynamic treatment of head and neck squamous cell carcinoma (HNSCC). Although it has a promising profile, its lipophilic character hampers the full exploitation of its potential due to high tendency of aggregation and a reduced ROS generation that compromise photodynamic therapy (PDT) efficacy. Moreover, for its clinical administration, mTHPC requires the presence of ethanol and propylene glycol as solvents, often causing adverse effects in the site of injection. In this paper we explored the efficiency of a new mTHPC formulation that uses human serum albumin (HSA) to disperse the photosensitizer in solution (mTHPC@HSA), investigating its anticancer potential in two HNSCC cell lines. Through a comprehensive characterization, we demonstrated that mTHPC@HSA is stable in physiological environment, does not aggregate, and is extremely efficient in PDT performance, due to its high singlet oxygen generation and the high dispersion as monomolecular form in HSA. This is supported by the computational identification of the specific binding pocket of mTHPC in HSA. Moreover, mTHPC@HSA-PDT induces cytotoxicity in both HNSCC cell lines, increasing intracellular ROS generation and the number of γ-H2AX foci, a cellular event involved in the global response to cellular stress. Taken together these results highlight the promising phototoxic profile of the complex, prompting further studies to assess its clinical potential.

Photosensitizing properties and subcellular localisation of 3,4-dihydro-β-carbolines harmaline and harmalol

Harmaline (1) and harmalol (2) represent two 3,4-dihydro-β-carboline (DHβCs) most frequently reported in a vast number of living systems. Fundamental aspects including the photosensitizing properties, cellular uptake, as well as the cyto- and phototoxicity of 1 and 2 were investigated herein. The molecular basis underlying the investigated processes are elucidated. Data reveal that both alkaloids show a distinctive pattern of extracellular DNA photodamage. Compound 1 induces a DNA photodamage profile dominated by oxidised purines and sites of base loss (AP sites), whereas 2 mostly induces single-strand breaks (SSBs) in addition to a small extent of purine oxidative damage. In both cases, DNA oxidative damage would occur through type I mechanism. In addition, a concerted hydrolytic attack is suggested as an extra mechanism accounting for the SSBs formation photoinduced by 2. Subcellular internalisation, cyto- and phototoxicity of 1 and 2 and the corresponding full-aromatic derivatives harmine (3) and harmol (4) also showed quite distinctive patterns in a structure-dependent manner. These results are discussed in the framework of the potential biological, biomedical and/or pharmacological roles reported for these alkaloids.

Graphical abstract

The subtle structural difference (i.e., the exchange of a methoxy group for a hydroxyl substituent at C(7)) between harmaline and harmalol, gives rise to distinctive photosensitizing and subcellular localisation patterns.

Enhanced Uptake and Phototoxicity of C60@albumin Hybrids by Folate Bioconjugation

Fullerenes are considered excellent photosensitizers, being highly suitable for photodynamic therapy (PDT). A lack of water solubility and low biocompatibility are, in many instances, still hampering the full exploitation of their potential in nanomedicine. Here, we used human serum albumin (HSA) to disperse fullerenes by binding up to five fullerene cages inside the hydrophobic cavities. Albumin was bioconjugated with folic acid to specifically address the folate receptors that are usually overexpressed in several solid tumors. Concurrently, tetramethylrhodamine isothiocyanate, TRITC, a tag for imaging, was conjugated to C₆₀@HSA in order to build an effective phototheranostic platform. The in vitro experiments demonstrated that: (i) HSA disperses C₆₀ molecules in a physiological environment, (ii) HSA, upon C₆₀ binding, maintains its biological identity and biocompatibility, (iii) the C₆₀@HSA complex shows a significant visible-light-induced production of reactive oxygen species, and (iv) folate bioconjugation improves both the internalization and the PDT-induced phototoxicity of the C₆₀@HSA complex in HeLa cells.

Overcoming barriers in photodynamic therapy harnessing nano-formulation strategies

Photodynamic therapy (PDT) has been extensively investigated for decades for tumor treatment because of its non-invasiveness, spatiotemporal selectivity, lower side-effects, and immune activation ability. It can be a promising treatment modality in several medical fields, including oncology, immunology, urology, dermatology, ophthalmology, cardiology, pneumology, and dentistry. Nevertheless, the clinical application of PDT is largely restricted by the drawbacks of traditional photosensitizers, limited tissue penetrability of light, inefficient induction of tumor cell death, tumor resistance to the therapy, and the severe pain induced by the therapy. Recently, various photosensitizer formulations and therapy strategies have been developed to overcome these barriers. Significantly, the introduction of nanomaterials in PDT, as carriers or photosensitizers, may overcome the drawbacks of traditional photosensitizers. Based on this, nanocomposites excited by various light sources are applied in the PDT of deep-seated tumors. Modulation of cell death pathways with co-delivered reagents promotes PDT induced tumor cell death. Relief of tumor resistance to PDT with combined therapy strategies further promotes tumor inhibition. Also, the optimization of photosensitizer formulations and therapy procedures reduces pain in PDT. Here, a systematic summary of recent advances in the fabrication of photosensitizers and the design of therapy strategies to overcome barriers in PDT is presented. Several aspects important for the clinical application of PDT in cancer treatment are also discussed.

N-Methyl-β-carboline alkaloids: structure-dependent photosensitizing properties and localization in subcellular domains

N-Methyl-β-carboline (βC) alkaloids, including normelinonine F (1b) and melinonine F (2b), have been found in a vast range of living species playing different biological, biomedical and/or pharmacological roles. Despite this, molecular bases of the mechanisms through which these alkaloids would exert their effect still remain unknown. Fundamental aspects including the photosensitizing properties and intracellular internalization of a selected group of N-methyl-βC alkaloids were investigated herein. Data reveal that methylation of the βC main ring enhances its photosensitizing properties either by increasing its binding affinity with DNA as a biomolecular target and/or by increasing its oxidation potential, in a structure-dependent manner. As a general rule, N(9)-substituted βCs showed the highest photosensitizing efficiency. With the exception of 2-methyl-harminium, all the N-methyl-βCs investigated herein induce a similar DNA photodamage profile, dominated largely by oxidized purines. This fact represents a distinctive behavior when comparing with N-unsubstituted-βCs. On the other hand, although all the investigated compounds might accumulate mainly into the mitochondria of HeLa cells, methylation provides a distinctive dynamic pattern for mitochondrial uptake. While rapid (passive) diffusion is most probably reponsible for the prompt uptake/release of neutral βCs, an active transport appears to mediate the (reatively slow) uptake of the quaternary cationic βCs. This might be a consequence of a distinctive subcellular localization (mitochondrial membrane and/or matrix) or interaction with intracellular components. Biomedical and biotechnological implications are also discussed herein.

Light-induced full aromatization and hydroxylation of 7-methoxy-1-methyl-3,4-dihydro-2H-pyrido[3,4-b]indole alkaloid: Oxygen partial pressure as a key modulator of the photoproducts distribution

Full-aromatic and partially hydrogenated β-carboline (βC) derivatives constitute a group of alkaloids widely distributed in a great variety of living systems. In plants and bacteria, tetrahydro-βCs are the primary product of the Pictet-Spengler enzymatically catalyzed condensation. Tetrahydro-βC skeleton is further modified giving rise to the formation of a vast set of derivatives including dihydro- and full-aromatic βCs. However, in most of the cases, the later processes still remain unclear and other sources, such as photo-triggered reactions, deserve to be explored. In this context, the photophysic and photochemistry of 7-methoxy-1-methyl-3,4-dihydro-2H-pyrido[3,4-b]indole or harmaline (Hlina) in aqueous solution is reported herein. UV-visible absorption and fluorescence emission spectroscopy coupled with multivariate data analysis (PARAFAC), HPLC and HRESI-MS techniques were used for both quantitative and qualitative analysis. The formation singlet oxygen and hydrogen peroxide reactive oxygen species (ROS) was quantified and their role together with the influence of pH and oxygen partial pressure on the photochemical degradation of HlinaH⁺ was assessed. We report herein the first study on photochemical full-aromatization of a dihydro-βC derivative. These results can shed some light on the βCs biosynthesis and role in living systems.

Folate-Decorated Amphiphilic Cyclodextrins as Cell-Targeted Nanophototherapeutics

Nowadays, active targeting of nanotherapeutics is a challenging issue. Here, we propose a rational design of a ternary nanoassembly (SAP) composed of nonionic amphiphilic β-cyclodextrins (amphiphilic CD) incorporating pheophorbide (Pheo) as a phototherapeutic and an adamantanyl-folic acid conjugate (Ada-FA) to target tumor cells overexpressing α-folate receptor (FR-α(+)). Dynamic light scattering and ζ-potential pointed out the presence of nanoassemblies bearing a negative surface charge (ζ = -51 mV). Morphology of SAP was investigated by atomic force microscopy and microphotoluminescence, indicating the presence of highly emissive near-spherical assemblies of about 280 nm in size. Complementary spectroscopic techniques such as ROESY-NMR, UV/vis and steady-state fluorescence revealed that the folic acid protrudes out of amphiphilic CD rims, prone for recognition with FR-α. Pheo was strongly loaded in the nanoassembly mostly in monomeric form, thus generating singlet oxygen (¹O₂) and consequentely showing phototherapeutic action. SAP remained stable until 2 weeks in aqueous solutions. Stability studies in biologically relevant media pointed out the ability of SAP to interact with serum proteins by means of the oligoethylenglycole fringe, without destabilization. Release experiments demonstrated the sustained release of Pheo from SAP in environments mimiking physiological conditions (∼20% within 1 week), plausibly suggesting low Pheo leaking and high integrity of the assembly within 24 h, time spent on average to reach the target sites. Cellular uptake of SAP was confirmed by confocal microscopy, pointing out that SAP was internalized into the tumoral cells expressing FR-α more efficiently than SP. SAP showed improved phototoxicity in human breast MCF-7 cancer cells FR-α(+) (IC₅₀ = 270 nM) with respect to human prostate carcinoma PC3 cells (IC₅₀ = 700 nM) that express a low level of that receptor (FR-α(-)). Finally, an improved phototoxicity in FR-α(+) MCF-7 cells (IC₅₀ = 270 nM) was assessed after treatment with SAP vs SP (IC₅₀ = 600 nM) which was designed without Ada-FA as a targeting unit.

Receptor-mediated Uptake of Folic Acid-functionalized Dextran Nanoparticles for Applications in Photodynamic Therapy

In photodynamic therapy (PDT), photosensitizers and light are used to cause photochemically induced cell death. The selectivity and the effectiveness of the phototoxicity in cancer can be increased by a specific uptake of the photosensitizer into tumor cells. A promising target for this goal is the folic acid receptor α (FRα), which is overexpressed on the surface of many tumor cells and mediates an endocytotic uptake. Here, we describe a polysaccharide-based nanoparticle system suitable for targeted uptake and its photochemical and photobiological characterization. The photosensitizer 5, 10, 15, 20-tetraphenyl-21H, 23H-porphyrine (TPP) was encapsulated in spermine- and acetal-modified dextran (SpAcDex) nanoparticles and conjugated with folic acid (FA) on the surface [SpAcDex(TPP)-FA]. The particles are successfully taken up by human HeLa-KB cells, and a light-induced cytotoxicity is observable. An excess of free folate as the competitor for the FRα-mediated uptake inhibits the phototoxicity. In conclusion, folate-modified SpAcDex particles are a promising drug delivery system for a tumor cell targeted photodynamic therapy.

Norharmane as a potential chemical entity for development of anticancer drugs

Cancer is a leading cause of death generally, and to overcome this problem the introduction of a new drug developing is a continuous endeavour. An alkaloid, norharmane and its derivatives, which have anticancer activities, widely distributed in several living and synthetic chemical sources. Herewith, the suggested mechanisms of organic reactions and synthetic approaches of norharmane available so far were considered. Active sites of norharmane nucleus positions, C-1, C-3, and N-9, were used for developing new molecules and based on structure activity relationship (SAR), those have been seen with anticancer activities. This review summarizes on chemistry of synthetic strategies of norharmane derivatives, which may provide a framework to design a novel anticancer drug, in future.

DNA damage photo-induced by chloroharmine isomers: hydrolysis versus oxidation of nucleobases

Photodynamic therapy (PDT) is an emerging clinical treatment currently being used against a wide range of both cancerous and noncancerous diseases. The search for new active photosensitizers as well as the development of novel selective delivery systems are the major challenges faced in the application of PDT. We investigated herein three chloroharmine derivatives (6-, 8- and 6,8-dichloroharmines) with quite promising intrinsic photochemical tunable properties and their ability to photoinduce DNA damage in order to elucidate the underlying photochemical mechanisms. Data revealed that the three compounds are quite efficient photosensitizers. The overall extent of photo-oxidative DNA damage induced by both 8-chloro-substituted β-carbolines is higher than that induced by 6-chloro-harmine. The predominant type of lesion generated also depends on the position of the chlorine atom in the β-carboline ring. Both 8-chloro-substituted β-carbolines mostly oxidize purines via type I mechanism, whereas 6-chloro-harmine mainly behaves as a "clean" artificial photonuclease inducing single-strand breaks and site of base loss via proton transfer and concerted (HO^--mediated) hydrolytic attack. The latter finding represents an exception to the general photosensitizing reactions and, to the best of our knowledge, this is the first time that this process is well documented. The controlled and selective production of different oxygen-independent lesions could be fine-tuned by simply changing the substituent groups in the β-carboline ring. This could be a promising tool for the design and development of novel photo-therapeutic agents aimed to tackle hypoxic conditions shown in certain types of tumours.

Development of a Novel Covalent Folate-Albumin-Photosensitizer Conjugate

There is considerable interest in the development of novel and more efficient delivery systems for improving the efficacy of photodynamic therapy (PDT). The authors in this highlighted issue describe the synthesis and the photobiological characterizations of two photosensitizer (PS) conjugates based on β-carboline derivatives covalently conjugated to folic acid (FA) coupled to bovine serum albumin (BSA) as a carrier system specifically targeting cancer cells overexpressing FA receptor alpha (FRα). Accordingly, only the FA-BSA-β-carboline conjugates are internalized specifically in FRα-positive cells and are proved to be phototoxic. On the other hand, albumin-β-carboline conjugates without FA or β-carboline derivatives alone are not internalized and nontoxic. This conjugate is among the first to produce a conjugate composed of a PS and FA molecules that are directly conjugated to BSA. In addition, the in vitro studies are the first evidence that directly conjugated FA-BSA can be used as carriers to selectively enhance cytotoxicity by PDT relative to unmodified PS or nontargeted BSA-PS. This strategy is a positive step forward for the covalent design and construction of a photodynamic nanomedicine for FR-positive tumors.