In vitro cellular uptake and dimerization of signal transducer and activator of transcription-3 (STAT3) identify the photosensitizing and imaging-potential of isomeric photosensitizers derived from chlorophyll-a and bacteriochlorophyll-a

Theranostics with photodynamic therapy for personalized medicine: to see and to treat

Photodynamic Therapy (PDT) is an approved treatment modality, which is presently receiving great attention due to its limited invasiveness, high selectivity and limited susceptibility to drug resistance. Another related research area currently expanding rapidly is the development of novel theranostic agents based on the combination of PDT with different imaging technologies, which allows for both therapy and diagnosis. This combination can help to address issues of suboptimal biodistribution and selectivity through regional imaging, while therapeutic agents enable an effective and personalized therapy. In this review, we describe compounds, whose structures combine PDT photosensitizers with different imaging probes - including examples for near-infrared optical imaging, magnetic resonance imaging (MRI) and nuclear imaging (PET or SPECT), generating novel theranostic drug candidates. We have intentionally focused our attention on novel compounds, which have already been investigated preclinically in vivo in order to demonstrate the potential of such theranostic agents for clinical applications.

Conjugates of Tetrapyrrolic Macrocycles as Potential Anticancer Target-Oriented Photosensitizers

Photodynamic therapy is a minimally invasive treatment of tumors using photosensitizers, light, and reactive oxygen species, which can destroy cellular structures. With the development of photodynamic therapy, significant efforts have been made to create new efficient photosensitizers with improved delivery to cells, stability, and selectivity against cancer tissues. Naturally occurring tetrapyrrolic macrocycles, such as porphyrins and chlorins, are very attractive as photosensitizers, and their structural modification and conjugation with other biologically active molecules are promising approaches for creating new photosensitizers specifically targeting cancer cells. The present review aims to highlight recent developments in the design, preparation, and investigation of complex conjugates of tetrapyrrolic macrocycles, which can potentially be used as sensitizers for target-oriented photodynamic therapy of cancer. In this review, we discuss the structure, photodynamic effect, and anticancer activity of the following conjugates of tetrapyrrolic macrocycles: (1) conjugates obtained by modifying peripheral substituents in porphyrins and chlorins; (2) conjugates of porphyrins and chlorins with lipids, carbohydrates, steroids, and peptides; (3) conjugates of porphyrins and chlorins with anticancer drugs and some other biologically active molecules; (4) metal-containing conjugates. The question of how the conjugate structure affects its specificity, internalization, localization, and photoinduced toxicity within cancer cells is the focus of this review.

Selective Determination of Glutathione Using a Highly Emissive Fluorescent Probe Based on a Pyrrolidine-Fused Chlorin

We report the use of a carboxylated pyrrolidine-fused chlorin (TCPC) as a fluorescent probe for the determination of glutathione (GSH) in 7.4 pH phosphate buffer. TCPC is a very stable, highly emissive molecule that has been easily obtained from meso-tetrakis(4-methoxycarbonylphenyl) porphyrin (TCPP) through a 1,3-dipolar cycloaddition approach. First, we describe the coordination of TCPC with Hg(II) ions and the corresponding spectral changes, mainly characterized by a strong quenching of the chlorin emission band. Then, the TCPC-Hg²⁺ complex exhibits a significant fluorescence turn-on in the presence of low concentrations of the target analyte GSH. The efficacy of the sensing molecule was tested by using different TCPC:Hg²⁺ concentration ratios (1:2, 1:5 and 1:10) that gave rise to sigmoidal response curves in all cases with modulating detection limits, being the lowest 40 nM. The experiments were carried out under physiological conditions and the selectivity of the system was demonstrated against a number of potential interferents, including cysteine. Furthermore, the TCPC macrocycle did not showed a significant fluorescent quenching in the presence of other metal ions.

Tumor cell-specific retention of photosensitizers determines the outcome of photodynamic therapy for head and neck cancer

Pheophorbide-based photosensitizers have demonstrated tumor cell-specific retention. The lead compound 3-[1'-hexyloxyethyl]-2-devinylpyropheophorbide-a (HPPH) in a clinical trial for photodynamic therapy of head and neck cancer lesions indicated a complete response in 80% of patients. The question arises whether the partial response in 20% of patients is due to inefficient retention of photosensitizers by tumor cells and, if so, can the photosensitizer preference of individual cancer cases be identified prior to photodynamic therapy. This study determined the specificity of head and neck cancer cells and tumor tissues for the uptake and retention of diffusible pheophorbides differing in peripheral groups on the macrocycle that contribute to cellular binding. The relationship between photosensitizer level and light-mediated photoreaction was characterized to identify markers for predicting the effectiveness of photodynamic therapy in situ. The experimental models were stromal and epithelial cells isolated from head and neck tumor samples and integrated into monotypic tissue cultures, reconstituted three-dimensional co-cultures, and xenografts. Tumor cell-specific photosensitizer retention patterns were identified, and a procedure was developed to allow the diagnostic evaluation of HPPH binding by tumor cells in individual cancer cases. The findings of this study may assist in designing conditions for photosensitizer application and photodynamic therapy of head and neck cancer lesions optimized for each patient's case.

Pathological Mechanism of Photodynamic Therapy and Photothermal Therapy Based on Nanoparticles

The ultimate goal of phototherapy based on nanoparticles, such as photothermal therapy (PTT) which generates heat and photodynamic therapy (PDT) which not only generates reactive oxygen species (ROS) but also induces a variety of anti-tumor immunity, is to kill tumors. In addition, due to strong efficacy in clinical treatment with minimal invasion and negligible side effects, it has received extensive attention and research in recent years. In this paper, the generations of nanomaterials in PTT and PDT are described separately. In clinical application, according to the different combination pathway of nanoparticles, it can be used to treat different diseases such as tumors, melanoma, rheumatoid and so on. In this paper, the mechanism of pathological treatment is described in detail in terms of inducing apoptosis of cancer cells by ROS produced by PDT, immunogenic cell death to provoke the maturation of dendritic cells, which in turn activate production of CD4+ T cells, CD8+T cells and memory T cells, as well as inhibiting heat shock protein (HSPs), STAT3 signal pathway and so on.

Synthesis and evaluation of novel chlorophyll a derivatives as potent photosensitizers for photodynamic therapy

Chlorophyll a exhibits excellent photosensitive activity in photosynthesis. The unstability limited its application as photoensitizer drug in photodynamic therapy. Here a series of novel chlorophyll a degradation products pyropheophorbide-a derivatives were synthesized and evaluated for lung cancer in PDT. These compounds have strong absorption in 660-670 nm with high molar extinction coefficient, and fluorescence emission in 660-675 nm upon excitation with 410-415 nm light. They all have much higher ROS yields than pyropheophorbide-a, and compound 10 was even higher than [3-(1-hexyloxyethyl)]-pyrophoeophorbide a (HPPH). Distinctive phototoxicity was observed in vitro and the inhibition effect was in light dose-dependent and drug dose-dependent style. They can effectively inhibit the growth of lung tumor in vivo. Among them, compound 8 and 11 have outstanding photodynamic anti-tumor effects without obvious skin photo-toxicity, so they can act as new drug candidates for photodynamic therapy.

Cell-specific Retention and Action of Pheophorbide-based Photosensitizers in Human Lung Cancer Cells

This study determined in primary cultures of human lung cancer cells the cell specificity of chlorin-based photosensitizers. Epithelial cells (ECs) preferentially retained 3-[1-hexyloxyethyl]-2-devinylpyropheophorbide-a (HPPH) and related structural variants. Tumor-associated fibroblasts (Fb) differ from EC by a higher efflux rate of HPPH. Immunoblot analyses indicated dimerization of STAT3 as a reliable biomarker of the photoreaction. Compared to mitochondria/ER-localized photoreaction by HPPH, the photoreaction by lysosomally targeted HPPH-lactose showed a trend toward lower STAT3 cross-linking. Lethal consequence of the photoreaction differed between EC and Fb with the latter cells being more resistant. A survey of lung tumor cases indicated a large quantitative range by which EC retains HPPH. The specificity of HPPH retention defined in vitro could be confirmed in vivo in selected cases grown as xenografts. HPPH retention as a function of the tetrapyrrole structure was evaluated by altering side groups on the porphyrin macrocycle. The presence or absence of a carboxylic acid at position 17² proved to be critical. A benzyl group at position 20 enhanced retention in a subset of cancer cells with low HPPH binding. This study indicated experimental tools that are potentially effective in defining the photosensitizer preference and application for individual patient's cancer lesions.

Highly Effective Dual-Function Near-Infrared (NIR) Photosensitizer for Fluorescence Imaging and Photodynamic Therapy (PDT) of Cancer

We report herein the synthesis and biological efficacy of near-infrared (NIR), bacteriochlorin analogues: 3-(1'-butyloxy)ethyl-3-deacetyl-bacteriopurpurin-18-N-butylimide methyl ester (3) and the corresponding carboxylic acid 10. In in vitro assays, compared to its methyl ester analogue 3, the corresponding carboxylic acid derivative 10 showed higher photosensitizing efficacy. However, due to drastically different pharmacokinetics in vivo, the PS 3 (HPLC purity >99%) showed higher tumor uptake and long-term tumor cure than 10 (HPLC purity >96.5%) in BALB/c mice bearing Colon 26 tumors. Isomerically pure R- and S- isomers of 3 (3a and 3b, purity by HPLC > 99%) under similar treatment parameters showed identical efficacy in vitro and in vivo. In addition, photosensitizer (PS) 3 showed limited skin phototoxicity and provides an additional advantage over the clinically approved chemically complex hematoporphyrin derivative as well as other porphyrin-based PDT agents, which makes 3 a promising dual-function agent for fluorescence-guided surgery with an option of phototherapy of cancer.

Monitoring Tumor Hypoxia Using (18)F-FMISO PET and Pharmacokinetics Modeling after Photodynamic Therapy

Photodynamic therapy (PDT) is an efficacious treatment for some types of cancers. However, PDT-induced tumor hypoxia as a result of oxygen consumption and vascular damage can reduce the efficacy of this therapy. Measuring and monitoring intrinsic and PDT-induced tumor hypoxia in vivo during PDT is of high interest for prognostic and treatment evaluation. In the present study, static and dynamic (18)F-FMISO PET were performed with mice bearing either U87MG or MDA-MB-435 tumor xenografts immediately before and after PDT at different time points. Significant difference in tumor hypoxia in response to PDT over time was found between the U87MG and MDA-MB-435 tumors in both static and dynamic PET. Dynamic PET with pharmacokinetics modeling further monitored the kinetics of (18)F-FMISO retention to hypoxic sites after treatment. The Ki and k3 parametric analysis provided information on tumor hypoxia by distinction of the specific tracer retention in hypoxic sites from its non-specific distribution in tumor. Dynamic (18)F-FMISO PET with pharmacokinetics modeling, complementary to static PET analysis, provides a potential imaging tool for more detailed and more accurate quantification of tumor hypoxia during PDT.

Intraoperative optical assessment of photodynamic therapy response of superficial oral squamous cell carcinoma

This study investigated whether diffuse optical spectroscopy (DOS) measurements could assess clinical response to photodynamic therapy (PDT) in patients with head and neck squamous cell carcinoma (HNSCC). In addition, the correlation between parameters measured with DOS and the crosslinking of signal transducer and activator of transcription 3 (STAT3), a molecular marker for PDT-induced photoreaction, was investigated. Thirteen patients with early stage HNSCC received the photosensitizer 2-[1-hexyloxyethyl]-2-devinylpyropheophorbide-a (HPPH) and DOS measurements were performed before and after PDT in the operating room (OR). In addition, biopsies were acquired after PDT to assess the STAT3 crosslinking. Parameters measured with DOS, including blood volume fraction, blood oxygen saturation (StO2), HPPH concentration (cHPPH), HPPH fluorescence, and blood flow index (BFI), were compared to the pathologic response and the STAT3 crosslinking. The best individual predictor of pathological response was a change in cHPPH (sensitivity=60%, specificity=100%), while discrimination analysis using a two-parameter classifier (change in cHPPH and change in StO2) classified pathological response with 100% sensitivity and 100% specificity. BFI showed the best correlation with the crosslinking of STAT3. These results indicate that DOS-derived parameters can assess the clinical response in the OR, allowing for earlier reintervention if needed.

Effect of chirality on cellular uptake, imaging and photodynamic therapy of photosensitizers derived from chlorophyll-a

We have previously shown that the (124)I-analog of methyl 3-(1'-m-iodobenzyloxy) ethyl-3-devinyl-pyropheophorbide-a derived as racemic mixture from chlorophyll-a can be used for PET (positron emission tomography)-imaging in animal tumor models. On the other hand, as a non-radioactive analog, it showed excellent fluorescence and photodynamic therapy (PDT) efficacy. Thus, a single agent in a mixture of radioactive ((124)I-) and non-radioactive ((127)I) material can be used for both dual-imaging and PDT of cancer. Before advancing to Phase I human clinical trials, we evaluated the activity of the individual isomers as well as the impact of a chiral center at position-3(1) in directing in vitro/in vivo cellular uptake, intracellular localization, epithelial tumor cell-specific retention, fluorescence/PET imaging, and photosensitizing ability. The results indicate that both isomers (racemates), either as methyl ester or carboxylic acid, were equally effective. However, the methyl ester analogs, due to subcellular deposition into vesicular structures, were preferentially retained. All derivatives containing carboxylic acid at the position-17(2) were noted to be substrate for the ABCG2 (a member of the ATP binding cassette transporters) protein explaining their low retention in lung tumor cells expressing this transporter. The compounds in which the chirality at position-3 has been substituted by a non-chiral functionality showed reduced cellular uptake, retention and lower PDT efficacy in mice bearing murine Colon26 tumors.

Polyacrylamide-based biocompatible Nanoplatform enhances the tumor uptake, PET/fluorescence imaging and anticancer activity of a chlorophyll analog

In this report we demonstrate the outstanding advantages of multifunctional nanoplatforms for cancer-imaging and therapy. The non-toxic polyacrylamide (PAA) nanoparticles (size:18-25 nm) formulation drastically changed the pharmacokinetic profile of the ¹²⁴I- labeled chlorophyll-a derivative (formulated in 10% ethanol in PBS) with a remarkable enhancement in tumor uptake, and significantly reduced uptake in spleen and liver. Among the various nanoformulations investigated, the ¹²⁴I- labeled photosensitizer (dose: 0.6142 MBq), and the cyanine dye-nanoparticles (CD-NP) conjugate (dose 0.3 μmol/kg) in combination showed great potential for tumor imaging (PET/NIR fluorescence) in BALB/c mice bearing Colon26 tumors. Compared to free non-labeled photosensitizer, the corresponding PAA nanoformulation under similar treatment parameters showed a remarkable enhancement in long-term tumor cure by PDT (photodynamic therapy) and provides an opportunity to develop a single nanoplatform for tumor-imaging (PET/fluorescence) and phototherapy, a practical "See and Treat" approach.

Radiolabelled porphyrins in nuclear medicine

Amongst tumour-specific substances, hematoporphyrin and synthetic porphyrin derivatives have been widely investigated to identify and delineate neoplastic and malignant tissue. Whilst the tumour localization exhibited by selected porphyrin species has been exploited through photodynamic therapy, several examples of porphyrin derivatives with varied peripheral functionality have been radiolabelled with the aim of developing porphyrin-based nuclear imaging and therapeutic agents. In this review, we look at the approaches and advances in the preparation and uses of such radiolabelled agents for imaging and therapy.

Multifunctional bacteriochlorins from selective palladium-coupling reactions

Nonsymmetrical, multifunctional bacteriochlorin derivatives possessing different substituents at the β-pyrrolic positions have been prepared by stepwise, selective functionalization of 3,13-dibromo-5-methoxybacteriochlorin via palladium-coupling reactions. The new derivatives reported here include monovalent bioconjugatable bacteriochlorin, orthogonally protected bacteriochlorin amino acid, and push-pull bacteriochlorins. Taken together, this study provides a route to previously unavailable bacteriochlorin architectures for fundamental studies and diverse applications.

Cell-type selective phototoxicity achieved with chlorophyll-a derived photosensitizers in a co-culture system of primary human tumor and normal lung cells

The ATP-dependent transporter ABCG2 exports certain photosensitizers (PS) from cells, implying that the enhanced expression of ABCG2 by cancer cells may confer resistance to photodynamic therapy (PDT) mediated by those PS. In 35 patient-derived primary cultures of lung epithelial and stromal cells, PS with different subcellular localization and affinity for ABCG2 displayed cell-type specific retention both independent and dependent on ABCG2. In the majority of cases, the ABCG2 substrate 2-[1-hexyloxyethyl]-2-devinyl pyropheophorbide-a (HPPH) was lost from fibroblastic cells more rapidly than from their epithelial counterparts, even in the absence of detectable ABCG2 expression, facilitating selective eradication by PDT of epithelial over fibroblastic cells in tumor/stroma co-cultures. Pairwise comparison of normal and transformed epithelial cells also identified tumor cells with elevated or reduced retention of HPPH, depending on ABCG2. Enhanced ABCG2 expression led to the selective PDT survival of tumor cells in tumor/stroma co-cultures. This survival pattern was reversible through HPPH derivatives that are not ABCG2 substrates or the ABCG2 inhibitor imatinib mesylate. PS retention, not differences in subcellular distribution or cell signaling responses, was determining cell type selective death by PDT. These data suggest that up-front knowledge of tumor characteristics, specifically ABCG2 status, could be helpful in individualized PDT treatment design.