Kinetics of porphyrin accumulation in cultured epithelial cells exposed to ALA

Targeting ABCG2 transporter to enhance 5-aminolevulinic acid for tumor visualization and photodynamic therapy

5-Aminolevulinic acid (ALA) has been approved by the U. S. FDA for fluorescence-guided resection of high-grade glioma and photodynamic therapy (PDT) of superficial skin precancerous and cancerous lesions. As a prodrug, ALA administered orally or topically is metabolized in the heme biosynthesis pathway to produce protoporphyrin IX (PpIX), the active drug with red fluorescence and photosensitizing property. Preferential accumulation of PpIX in tumors after ALA administration enables the use of ALA for PpIX-mediated tumor fluorescence diagnosis and PDT, functioning as a photo-theranostic agent. Extensive research is currently underway to further enhance ALA-mediated PpIX tumor disposition for better tumor visualization and treatment. Particularly, the discovery of PpIX as a specific substrate of ATP binding cassette subfamily G member 2 (ABCG2) opens the door to therapeutic enhancement with ABCG2 inhibitors. Studies with human tumor cell lines and human tumor samples have demonstrated ABCG2 as an important biological determinant of reduced ALA-PpIX tumor accumulation, inhibition of which greatly enhances ALA-PpIX fluorescence and PDT response. These studies strongly support targeting ABCG2 as an effective therapeutic enhancement approach. In this review, we would like to summarize current research of ABCG2 as a drug efflux transporter in multidrug resistance, highlight previous works on targeting ABCG2 for therapeutic enhancement of ALA, and provide future perspectives on how to translate this ABCG2-targeted therapeutic enhancement strategy from bench to bedside.

Selective 5-aminolevulinic acid-induced protoporphyrin IX fluorescence in Gliomas

Malignant gliomas are locally invasive tumors that offer a poor prognosis. Evidence shows that complete resection of the tumor at the time of surgery confers a significant improvement in overall survival. In recent years, 5- aminolevulinic acid (ALA)-induced fluorescence has been used by neurosurgeons to good effect in increasing the rate of complete resection. Despite the considerable interest in the use of 5-ALA in fluorescence-guided neurosurgery, the mechanisms behind the accumulation of Protoporphyrin IX (PpIX) in neoplastic tissue are unclear. In this review, we summarize the evidence in the literature on the mechanisms underlying the selective production of PpIX with a specific focus on gliomas.

Factors implicated in the assessment of aminolevulinic acid-induced protoporphyrin IX fluorescence

BACKGROUND

Photodynamic therapy and photodiagnosis of cancer requires preferential accumulation of fluorescent photosensitizers in tumors. Clinical evidence documents feasibility of ALA-based photodiagnosis for tumor detection. However, false positive results and large variations in fluorescence intensities are also reported. Furthermore, selective accumulation of fluorescent species of photosensitizers in tumor cell lines, as compared to normal ones, when cultured in vitro, is not always observed. To understand this discrepancy we analyzed the impact of various factors on the intensity of detected PpIX fluorescence.

METHODS

Impacts of cell type, mitochondrial potential, cell-cell interactions and relocalization of PpIX among different cell types in co-cultures of different cell lines were analyzed by confocal microscopy and flow cytometry. Fluorescence spectroscopy was used to estimate absolute amounts of ALA-induced PpIX in individual cell lines. Immunofluorescence staining was applied to evaluate the ability of cell lines to produce collagen.

RESULTS

Higher ALA-induced PpIX fluorescence in cancer cell lines as compared to normal ones was not detected by all the methods used. Mitochondrial activity was heterogeneous throughout the cell monolayers and could not be clearly correlated with PpIX fluorescence. Positive collagen staining was detected in all cell lines tested.

CONCLUSIONS

Contrary to in vivo situation, ALA-induced PpIX production by cell lines in vitro may not result in higher PpIX fluorescence signals in tumor cells than in normal ones. We suggest that a combination of several properties of tumor tissue, instead of tumor cells only, is responsible for increased ALA-induced PpIX fluorescence in solid tumors.

GENERAL SIGNIFICANCE

Understanding the reasons of increased ALA-induced PpIX fluorescence in tumors is necessary for reliable ALA-based photodiagnosis, which is used in various oncological fields.

Serum-dependent export of protoporphyrin IX by ATP-binding cassette transporter G2 in T24 cells

Accumulation of protoporphyrin IX (PpIX) in cancer cells is a basis of 5-aminolevulinic acid (ALA)-induced photodymanic therapy. We studied factors that affect PpIX accumulation in human urothelial carcinoma cell line T24, with particular emphasis on ATP-binding cassette transporter G2 (ABCG2) and serum in the medium. When the medium had no fetal bovine serum (FBS), ALA induced PpIX accumulation in a time- and ALA concentration-dependent manner. Inhibition of heme-synthesizing enzyme, ferrochelatase, by nitric oxide donor (Noc18) or deferoxamine resulted in a substantial increase in the cellular PpIX accumulation, whereas ABCG2 inhibition by fumitremorgin C or verapamil induced a slight PpIX increase. When the medium was added with FBS, cellular accumulation of PpIX stopped at a lower level with an increase of PpIX in the medium, which suggested PpIX efflux. ABCG2 inhibitors restored the cellular PpIX level to that of FBS(-) samples, whereas ferrochelatase inhibitors had little effects. Bovine serum albumin showed similar effects to FBS. Fluorescence microscopic observation revealed that inhibitors of ABC transporter affected the intracellular distribution of PpIX. These results indicated that ABCG2-mediated PpIX efflux was a major factor that prevented PpIX accumulation in cancer cells in the presence of serum. Inhibition of ABCG2 transporter system could be a new target for the improvement of photodynamic therapy.

Pharmacokinetics and selectivity of porphyrin synthesis after topical application of hexaminolevulinate in patients with cervical intraepithelial neoplasia

OBJECTIVE

Photodynamic therapy has the potential of a minimal invasive outpatient procedure for CIN patients at reproductive ages. The purpose of this study was to examine the pharmacokinetics and selectivity of porphyrin synthesis after topical application of hexaminolevulinate (HAL) in patients with cervical intraepithelial neoplasia (CIN).

STUDY DESIGN

Twenty-four women with CIN 1-3 received 4 mmol/L or 10 mmol/L HAL dissolved in thermolabile pluronic F 127 gel topically 5-720 minutes before surgical conization. The microscopic fluorescence pattern was quantified by ex vivo fluorescence spectroscopy at a wavelength of 635 nm and semiquantitatively by digital image analysis from cryosections.

RESULTS

With 4 and 10 mmol/L HAL, porphyrin fluorescence intensity increased over time, reaching a peak after 180-540 minutes application intervals in CIN and normal epithelium. However, the peak was much more pronounced in CIN than in normal tissues. The fluorescence intensity with 10 mmol/L was significantly higher than that with 4 mmol/L in CIN tissues (P < .05). Maximum intensity could generally be detected in the lower layer of the epithelium.

CONCLUSION

HAL seems to be a promising molecule for fluorescence diagnosis. For further treatment studies, we recommend application of 10 mmol/L HAL 300-540 minutes before photodynamic therapy in CIN.

5-Aminolevulinic acid-induced protoporphyrin-IX accumulation and associated phototoxicity in macrophages and oral cancer cell lines

Studies were carried out on 5-aminolevulinic acid (ALA)-induced protoporphyrin (PpIX) synthesis in mice peritoneal macrophages and two human oral squamous cell carcinoma (OSCC) cell lines NT8e and 4451. Cells were treated with 200 microg/ml ALA for 15 h and PpIX accumulation was monitored by spectrofluorometry and phototoxicity to red light (630+/-20 nm) was measured by MTT assay. PpIX accumulation was higher in macrophages as compared to OSCC cells under both normal serum concentration (10%) and conditions of serum depletion. The results on phototoxicity measurements correlated well with the levels of PpIX accumulation in both macrophages and cancer cells. While red light caused 20% phototoxicity in macrophages, no phototoxicity was seen in 4451 cells at 10% serum. Decrease in serum concentration to 5% and 1% led to higher phototoxicity corresponding to 40% and 70% in macrophages and 10% and 15% in 4451 cells. Similar results were obtained in NT8e cell line. Propidium iodide staining followed by fluorescence microscopic observations on photodynamically treated co-culture of murine or human macrophages and cancer cells showed selective damage to macrophages. These results suggest that in OSCC, macrophages would contribute more to tumor PpIX level than tumor cells themselves and PDT may lead to selective killing of macrophages at the site of treatment. Since macrophages are responsible for production and secretion of various tumor growth mediators, the effect of selective macrophage killing on the outcome of PDT would be significant.

Fluorescence diagnosis in keratinocytic intraepidermal neoplasias

BACKGROUND

As different tissue types have distinct capabilities to accumulate protoporphyrin-IX, fluorescence diagnosis with aminolevulinic acid-induced porphyrin (FDAP) could be used to discriminate between different tissue types.

OBJECTIVE

Protoporphyrin-IX accumulation and proliferation were studied in cutaneous squamous (pre)malignancies to see whether FDAP could be used to discriminate between different stages of keratinocytic intraepidermal neoplasia or proliferative status.

METHODS

FDAP was performed in 14 patients (86 lesions) and biopsy specimens were taken, on which (immuno)histochemistry was performed for histopathologic classification and assessment of Ki67-antigen expression. Stratum corneum thickness was also measured.

RESULTS

The fluorescence ratio (lesional:nonlesional skin) showed neither significant differences between the different keratinocytic intraepidermal neoplasia stages, nor between different levels of Ki67-antigen expression. Macroscopic fluorescence intensity and stratum corneum thickness were negatively correlated.

LIMITATIONS

Relatively few malignancies were biopsied.

CONCLUSIONS

With FDAP we were not able to discriminate between keratinocytic intraepidermal neoplasia lesions or proliferative activity. However, hyperkeratosis appeared to be an important determinant in variations in macroscopic fluorescence intensity.

High energy shock waves activate 5'-aminolevulinic Acid and increase permeability to Paclitaxel: antitumor effects of a new combined treatment on anaplastic thyroid cancer cells

OBJECTIVE

Multimodal treatments do not meaningfully improve survival of anaplastic thyroid cancer. Consequently, new effective therapeutic modalities are needed. The use of paclitaxel is under clinical investigation; it shows about a 50% response rate, but it is not able to alter the fatal outcome for patients with anaplastic carcinoma. High energy shock waves (HESW) have been shown to cause a transient increase in the permeability of cell membranes thus allowing higher intracellular drug concentrations. 5-Aminolevulinic acid (ALA) is used in the photodynamic therapy (PDT) of cancer, and HESW are under evaluation for their use as an activator in ALA-PDT.

DESIGN

We investigated the effect of HESW produced by a piezoelectric generator on the sensitivity to paclitaxel and ALA treatments of two different anaplastic thyroid cancer cell lines (ARO and CAL-62). Cells, treated sequentially with ALA and paclitaxel were exposed to HESW; thereafter, cell viability and apoptosis induction were evaluated.

MAIN OUTCOME

Combined exposure to ALA, paclitaxel, and shock waves resulted in a significant enhancement of cytotoxicity and induction of apoptosis in thyroid cancer cells with respect to cells treated with paclitaxel alone.

CONCLUSIONS

These preliminary data suggest the possibility of using HESW and ALA in combination with paclitaxel as a promising new therapy in the treatment of anaplastic thyroid cancer.

Fluoreszenzdiagnostik mit 5-ALA-Thermogel bei zervikaler intraepithelialer Neoplasie

OBJECTIVE

To study the pharmacokinetics and selectivity of 5-aminolevulinic acid (5-ALA) from 5-ALA thermolabile gel formulation in cervical intraepithelial neoplasia (CIN). The novel thermogel Pluronic F127 is liquid at cold temperatures and turns into a gel-like consistency at body temperature, thereby improving adhesion of 5-ALA to the cervix uteri.

METHODS

27 female patients with CIN 1-3 were included in this study. Thirty minutes to 12 h before conisation, 10 mL of thermo-gel containing either 4, 10, or 20% of 5-ALA were topically applied to the cervix. Biopsies were taken from the lesions as well as normal surrounding epithelial tissue for histological examination, fluorescence microscopy and spectrometry.

RESULTS

The thermogel Pluronic F127 was easy to handle and proved reliable as vehicle carrier. 5-ALA induced porphyrin fluorescence was maximal after application of 10% 5-ALA thermogel. We observed the higher porphyrin fluorescence intensity within the CIN lesions (1,116 +/- 241 AU), as compared to normal adjacent epithelium (704 +/- 166 AU). This difference was statistically significant (p < 0.05). Four to six hours after application, porphyrin fluorescence in dysplastic epithelium reached maximal intensity, and tumor selectivity was the highest in CIN 3 achieving a tumor-to-normal ratio of 3.5. Compared to other studies employing 5-ALA, the thermogel preparation of 5-ALA leads to twofold increase in the porphyrin selectivity.

CONCLUSIONS

The thermogel Pluronic F127 seems to improve the local cervical drug application. Based on these results, we recommend the application of a 10% 5-ALA thermogel formulation 4-6 h prior to photodynamic therapy in CIN.

Mechanistic studies on delta-aminolevulinic acid uptake and efflux in a mammary adenocarcinoma cell line

Delta-aminolevulinic acid (ALA) is the precursor in the biosynthesis of porphyrins. The knowledge of both the regulation of ALA entrance and efflux from the cells and the control of porphyrin biosynthesis is essential to improve ALA-mediated photodynamic therapy. In this work, we studied the regulation of ALA uptake and efflux by endogenously accumulated ALA and/or porphyrins in murine mammary adenocarcinoma cells. Under our set of conditions, the haem synthesis inhibitor succinyl acetone completely prevented porphobilinogen and porphyrin synthesis from ALA, and led to an increase in the intracellular ALA pool. However, neither intracellular ALA nor porphyrin pools regulate ALA uptake or efflux during the first 15 min of the process. Based on temperature dependence data, ALA but not gamma-aminobutyric acid (GABA) efflux is mediated by a diffusion mechanism. Moreover, the addition of extracellular GABA not only did not influence the rate of ALA efflux but on the contrary it affected ALA uptake, showing the contribution of a saturable mechanism for the uptake, but not for the efflux of ALA from the cells.

Cellular damage to human hepatocytes through repeated application of 5-aminolevulinic acid

BACKGROUND/AIMS

5-Aminolevulinic acid (ALA), a precursor of porphyrins is used for photodynamic diagnosis and therapy within topical or systemic applications. A potential toxic effect on the human liver is of major interest and therefore we investigated the impact of a repeated application of ALA without illumination on cultures of human hepatocytes.

METHODS

After ALA treatment of hepatocytes in vitro the porphyrin synthesis, albumin secretion, liver-specific enzyme release, and malondialdehyde levels were determined. In order to reduce levels of reactive oxygen substances, mannitol and the antioxidant enzymes superoxide dismutase and catalase were supplemented.

RESULTS

Porphyrin biosynthesis by human hepatocytes in vitro was repeatedly stimulated by ALA (0.001-1.0 mM), which was indicated by an accumulation of protoporphyrin IX. A repetitive treatment (up to four times) of hepatocytes with ALA resulted in an impairment of the hepatic function and viability, depending on the ALA concentration (0.1-1.0 mM) and frequency of application (2-3 times). This was also accompanied by increased malondialdehyde levels indicating enhanced lipid peroxidation. Only superoxide dismutase was able to reduce cellular damage and prevent specific function.

CONCLUSIONS

Repeated, not single, ALA treatment without illumination may cause deleterious effects to the liver, which are mediated by oxygen radicals and inhibited by an antioxidant.

Du temps perdu a la recherche

An attempt has been made to justify an academic career by outlining the approximate history of the development of ideas and tracing their origins and arborization. Because there are multifarious influences and formative factors involved in their genesis it has proved almost impossible to acknowledge all the important contributors to the patchwork of investigations that are summarized. However, whilst the review is incomplete and necessarily brief, it is hoped that a glimpse of some twentieth century preoccupations with biological science is afforded.

The 5-aminolevulinic acid-induced porphyrin biosynthesis in benign and malignant cells of the skin

In fluorescence diagnosis and photodynamic therapy of neoplastic tissues 5-aminolevulinic acid is used to synthesize endogenous porphyrins as photosensitizers. The efficacy of neoplastic tissues to fluorescence diagnosis and photodynamic therapy is thought to be dependent on the total level of intralesional formed porphyrins. The available profiles of porphyrin metabolites in normal and in neoplastic cell lines after administration of 5-aminolevulinic acid vary considerably. Thus, this is the first in-vitro study which compares the porphyrin biosynthesis in normal skin cells (HaCaT, fibroblasts) with melanoma cells (Bro, SKMel-23, SKMel-28). After incubation with 1 mM 5-aminolevulinic acid, kinetics of porphyrin levels and metabolites were determined in the cells and the corresponding supernatants. Exogenous 5-aminolevulinic acid induced porphyrin formation in all cells with maximum values after an incubation period of 16-36 h. Increase of porphyrin levels varied from 10- to 80-fold (SKMel-28>HaCaT>fibroblasts>SKMel-23>>Bro) with minimum 1.5 times higher levels of porphyrins in the supernatants than in the cells. In cells and supernatants protoporphyrin and coproporphyrin were the predominantly formed porphyrin metabolites. Metastatic melanoma cells (SKMel-23, SKMel-28) accumulated much higher porphyrin levels than primary melanoma cells (Bro). In conclusion, by optimizing the treatment modalities, especially the light source, topical photodynamic therapy (PDT) could become a treatment alternative of melanoma metastases in progressive disease.

A comparative study of normal and reverse phase high pressure liquid chromatography for analysis of porphyrins accumulated after 5-aminolaevulinic acid treatment of colon adenocarcinoma cells

Primary adenocarcinoma cells of the rectosigmoid colon (WiDr-cells) were treated with 5-aminolevulinic acid (5-ALA). Cellular porphyrins were separated and quantified by high performance liquid chromatography (HPLC), both as free porphyrin acids after an easy extraction method with a subsequent reverse phase technique, and then as porphyrin esters after a more laborious extraction method and subsequent normal phase technique. The porphyrins were detected by means of a fluorescence detector. Analysis by normal phase HPLC indicated that 81% (739 pmol/mg protein) of the total amounts of fluorescing porphyrins accumulated was protoporphyrin IX, while similar analysis by reverse phase HPLC indicated that PpIX constituted 91% (622 pmol/mg protein) of the accumulated porphyrins. In addition to protoporhyrin IX, copro-, hexa-, hepta- and uroporphyrins were observed in extracts from 5-ALA-treated cells by both methods. The discrepancy between the two methods increased with increasing hydrophilicity of the analysed porphyrins, with uroporphyrin estimated to be 6-fold higher (63 vs. 10 pmol/mg protein) by normal than by reverse phase HPLC.

Tissue distribution and kinetics of endogenous porphyrins synthesized after topical application of ALA in different vehicles

The use of 5-aminolaevulinic acid (ALA) is gaining increasing attention for photosensitization in photodynamic therapy of superficially localized tumours. The aim of this work was to determine the kinetics of porphyrin generation in tissues after topical application of ALA delivered in different vehicles on the skin overlying the tumour and normal skin of mice. Maximal accumulation was found in tumour 3 h after ALA application in both cream and lotion preparations. Normal and overlying tumour skin tissues showed different kinetic patterns, reflecting histological changes when the latter is invaded by tumour cells. Liver, kidney, spleen and blood porphyrins also raised from basal levels, showing that ALA and/or ALA-induced porphyrins reach all tissues after topical application. During the first 24 h of ALA topical application, precursors and porphyrins are excreted by both urine and faeces. ALA lotion applied on the skin overlying the tumour induced higher accumulation of tumoural porphyrins than cream, and lotion applied on normal skin appeared to be the most efficient upon inducing total body porphyrins. This work has demonstrated the great influence of the formulation of ALA vehicle on penetration through the skin. Knowledge of the kinetics of porphyrin generation after different conditions of ALA application is needed for the optimization of diagnosis and phototherapy in human tumours.

The iron regulatory protein can determine the effectiveness of 5-aminolevulinic acid in inducing protoporphyrin IX in human primary skin fibroblasts

The level of endogenous photosensitiser, protoporphyrin IX (PPIX), can be enhanced in the cells by 5-aminolevulinic acid (ALA). We investigated the effect of critical parameters such as growth state of the cells and availability of intracellular iron in modulating the level of PPIX, in human primary cultured skin fibroblasts (FEK4) maintained either in exponentially growing or growth-arrested phase, following treatment with ALA. The addition of ALA to exponentially growing cells increased the level of PPIX 6-fold relative to control cells; however, in growth-arrested cells the same treatment increased the level of PPIX up to 34-fold. The simultaneous addition of the hydrophilic iron-chelator Desferal with ALA, boosted the level of PPIX up to 47-fold in growing cells and up to 42-fold in growth-arrested cells, suggesting that iron is limiting under the latter conditions. The strict dependence of PPIX enhancement on free available iron levels was examined by the level of activation of iron regulatory protein in band shift assays. This analysis revealed that the basal level of iron regulatory protein in growth-arrested cells was 6-fold higher than in growing cells, reflecting the influence of the free available iron pool in exponentially growing cells. Interestingly, the same ratio was found between the basal level concentration of PPIX in growing and growth-arrested cells. We propose that iron regulatory protein activation could serve as a marker for developing photodynamic therapy protocols because it identifies cells and tissues with a propensity to accumulate PPIX and it is therefore likely to predict the effectiveness of such therapies.

Photodynamically induced effects in colon carcinoma cells (WiDr) by endogenous photosensitizers generated by incubation with 5-aminolaevulinic acid

Human adenocarcinoma cells of the line WiDr have been treated with 2 mM 5-aminolaevulinic acid (5-ALA) in the presence of 10% foetal calf serum. The treatment induces a linear accumulation of protoporphyrin IX (PpIX) for at least 7.5 h. After 7.5 h of incubation about 45% of the PpIX accumulated is cell-bound, while the rest is found in the medium (25%) or lost from the cells during washing with phosphate-buffered saline (30%). Exposure to white light at an intensity of 30 W/m2 for 18 min results in 95% reduction of clonogenicity in cells treated with 2 mM 5-ALA for 3.5 h. The enzymatic activities of enzymes located in cytosol (glyceraldehyde 3-phosphate dehydrogenase and lactate dehydrogenase) and lysosomes (acid phosphatase and beta-glucuronidase) are not influenced by a 5-ALA and light treatment inactivating about 35% of the cells. The MTT assay, which reflects mitochondrial dehydrogenase activity, but not succinate dehydrogenase, is partly inhibited by the same treatment. Treatment with 5-ALA in the absence of light increases O2 consumption by a factor of two, while the O2 consumption is inhibited when 5-ALA treatment is combined with exposure to light. In addition, 5-ALA and light exposure enhance accumulation of rhodamine 123 by 40% and reduce the intracellular ATP level by 25%. Confocal laser scanning microscopical analysis indicates granular perinuclear localization of the PpIX formed by 5-ALA treatment. In conclusion, photodynamic treatment using 5-ALA as a prodrug induces damage to mitochondrial function without inhibiting lysosomal and cytosolic marker enzymes.

Ultrastructural changes in PAM cells after photodynamic treatment with delta-aminolevulinic acid-induced porphyrins or photosan

Photodynamic therapy (PDT) is the combination of a photosensitizing drug (Ps) with light in the presence of oxygen leading to the generation of reactive molecular species and destruction of cancer cells. In this study we compared PDT with two Ps, the hematoporphyrin derivative Photosan (Ph) and delta-aminolevulinic acid (ALA)-induced endogenous protoporphyrin IX, with respect to mitochondrial function and ultrastructural alterations. The effects of PDT were investigated in PAM 212 cells after different Ps incubation times, light doses, and post-treatment periods. Both Ps induced a light dose-dependent impairment of the mitochondrial function with the dose-response curve being steep for ALA and flat for Ph. The prolongation of the incubation time from 4 to 20 h resulted in an increased reduction of mitochondrial activity after ALA PDT but not after Ph PDT. Treatment with an irradiation dose that decreased mitochondrial activity by 50% (IC50) led to early and profound changes of mitochondrial morphology in ALA photosensitized cells, whereas photosensitization with Ph resulted in more pronounced alterations of lysosomes. We conclude that at bioequivalent sublethal PDT exposures of PAM 212 cells, ALA-induced damage is primarily restricted to mitochondria, whereas Ph-induced cytotoxicity is mediated by damage of the lysosomal system.

Hypoxia significantly reduces aminolaevulinic acid-induced protoporphyrin IX synthesis in EMT6 cells

We have studied the effects of hypoxia on aminolaevulinic acid (ALA)-induced protoporphyrin IX (PpIX) synthesis in EMT6 monolayer cultures characterized by different cell densities and proliferation rates. Specifically, after ALA incubation under hypoxic or normoxic conditions, we detected spectrofluorometrically the PpIX content of the following populations: (a) low-density exponentially growing cells; (b) high-density fed-plateau cells; and (c) high-density unfed-plateau cells. These populations were selected either for the purpose of comparison with other in vitro studies (low-density exponentially growing cells) or as representatives of tumour regions adjacent to (high-density fed-plateau cells) and further away from (high-density unfed-plateau cells) capillaries. The amount of PpIX per cell produced by each one of these populations was higher after normoxic ALA incubation. The magnitude of the effect of hypoxia on PpIX synthesis was dependent on cell density and proliferation rate. A 42-fold decrease in PpIX fluorescence was observed for the high-density unfed-plateau cells. PpIX production by the low-density exponential cells was affected the least by ALA incubation under hypoxic conditions (1.4-fold decrease), whereas the effect on the high-density fed-plateau population was intermediate (20-fold decrease).

Simulations on the selectivity of 5-aminolaevulinic acid-induced fluorescence in vivo

The knowledge of the exact time course of a photosensitizer in tumour and surrounding host tissue is fundamental for effective photodynamic therapy (PDT) and fluorescence-based diagnosis. In this study the time course of porphyrin fluorescence following topical application of 5-aminolaevulinic acid (ALA) using different formulations, concentrations and incubation times has been measured in amelanotic melanomas (A-Mel-3) (n = 54) grown in transparent dorsal skinfold chambers of Syrian golden hamsters and in human basal cell carcinomas (BCCs) (n = 40) in vivo. To simulate the accumulation of ALA-induced protoporphyrin IX (Pp IX), a three-compartment model has been developed and rate constants have been determined. The kinetics of both the A-Mel-3 tumours and the BCCs show a significantly higher fluorescence intensity in tumour as compared to normal surrounding host tissue. Maximal fluorescence intensity in A-Mel-3 tumours as a percentage of the reference standard used occurs 150 min post incubation (p.i.) using a 1, 3 or 10% (vol.) ALA solution buffered to pH 7.4 and 1 h incubation time. After a 4 h incubation time maximal fluorescence intensity in tumour is measured shortly p.i. A concentration of 10% ALA does not increase the fluorescence intensity as compared to 3% ALA following 4 h incubation, but either 3 or 10% ALA yields a significantly higher fluorescence after 4 h incubation time as compared to 1 h. The fluorescence intensity following an 8 h incubation reaches its maximum directly p.i. for all concentrations and then decreases exponentially. The fluorescence intensity in the surrounding host tissue shows no statistically significant difference regarding concentration or incubation time. At least during the first hour p.i., the fluorescence intensity measured in the surrounding tissue is lower as compared to that in the tumour in all groups. 24 h after topical application hardly any fluorescence is detectable in tumour or surrounding host tissue in all experimental groups. Incubating human BCCs with a 20% ALA cream (water-in-oil emulsion) or a 20% ALA gel (containing 40% dimethyl sulfoxide) for approximately 2 h yields a similar fluorescence intensity directly after incubation for either cream or gel. However, while yielding a maximum 120 min p.i. with cream, the fluorescence intensity increases for a longer time (about 2-3 h p.i.) and up to higher values using the gel formulation. In surrounding normal skin, cream as well as gel formulation yields a similar fluorescence intensity directly after incubation. Afterwards the fluorescence intensity decreases slowly using the cream whereas a further increase of the fluorescence intensity is measured in the normal skin with a maximum 240 min p.i. using the gel formulation. The results of the proposed three-compartment model indicate that the observed selectivity of accumulated porphyrins following topical application of ALA is mainly governed by an increased ALA penetration of the stratum corneum of the skin, an accelerated ALA uptake into the cell and a higher porphyrin formation in tumour as compared to normal skin tissue, but not by a reduced ferrocheletase activity.

Cell cycle dependency of porphyrin uptake in a glioma cell line

We used a two-color analysis system to assess the porphyrin uptake and DNA content in four established cell lines of glioma employing flow cytometry (FCM). The FCM study revealed porphyrin uptake in all cells, regardless of the phase of the cell cycle they were in. However, those in the G0/G1 phase showed moderate uptake of porphyrin and those in the G2/M phase showed a higher uptake. These results indicated the advantage of using porphyrin as the carrier of tumor targeting agents as a therapeutic strategy for malignant tumors.

Cell cycle phase influences tumour cell sensitivity to aminolaevulinic acid-induced photodynamic therapy in vitro

Photodynamic therapy (PDT) is a form of cancer treatment based on the destruction of cells by the interaction of light, oxygen and a photosensitizer. Aminolaevulinic acid (ALA) is the prodrug of the photosensitizer protoporphyrin IX (PpIX). ALA-induced PDT depends on the rate of cellular synthesis of PpIX, which may vary with cell cycle phase. This study has investigated the relationship between cell cycle phase, PpIX generation and phototoxicity in synchronized and unsynchronized bladder cancer cells (HT1197). In unsynchronized cells, relative PpIX fluorescence values (arbitrary units) were significantly different between cell cycle phases after a 1-h ALA incubation (G1 24.8 +/- 0.7; S-phase, 32.7 +/- 0.8, P < 0.05; G2 35.4 +/- 0.8, P < 0.05). In synchronized cells after a 1-h ALA incubation, cells in G1 produced less PpIX than those in S-phase or G2 [6.65 +/- 1.1 ng per 10(5) cells compared with 15.5 +/- 2.1 (P < 0.05), and 8.1 +/- 1.8 ng per 10(5) cells (not significant) respectively] and were significantly less sensitive to ALA-induced PDT (% survival, G1 76.2 +/- 8.3; S-phase 49.7 +/- 4.6, P < 0.05; G2 44.2 +/- 2.4, P < 0.05). This differential response in tumour cells may have implications for clinical PDT, resulting in treatment resistance and possible failure in complete tumour response.

5-Aminolaevulinic acid-induced protoporphyrin IX accumulation in tissues: pharmacokinetics after oral or intravenous administration

In this study, the biodistribution of 5-aminolaevulinic acid (ALA) and accumulation of protoporphyrin IX (PpIX) in rats have been examined. Two groups of 21 WAG/Rij rats are given 200 mg/kg ALA orally or intravenously. Six rats serve as controls. At 1, 2, 3, 4, 6, 12 and 24 h after ALA administration, ALA and porphyrin concentrations are measured in 18 tissues and fluids. Liver enzymes and renal-function tests are measured to determine ALA toxicity. In both groups ALA concentration is highest in kidney, bladder and urine. After oral administration, high concentrations are also found in duodenal aspirate and jejunum. Mild, short-lasting elevation of creatinine is seen in both treatment groups. Porphyrins, especially PpIX, accumulate mainly in duodenal aspirate, jejunum, liver and kidney (> 10 nmol/g tissue), less in oesophagus, stomach, colon, spleen, bladder, heart, lung and nerve (2-10 nmol/g tissue), and only slightly in plasma, muscle, fat, skin and brain (< 2 nmol/g tissue). In situ synthesis of porphyrins rather than enterohepatic circulation contributes to the PpIX accumulation. Confocal laser scanning microscopy shows selective porphyrin fluorescence in epithelial layers. Peak levels and total production of porphyrins are equal after oral and intravenous ALA administration.

IN CONCLUSION

administration of 200 mg/kg ALA results in accumulation of photosensitive concentrations of PpIX, 1 to 6 h after ALA administration, in all tissues except muscle, fat, skin and brain. Knowledge of the time-concentration relationship should be helpful in selecting dosages, routes of administration and timing of ALA photodynamic therapy.

Protoporphyrin IX accumulation in cells treated with 5-aminolevulinic acid: dependence on cell density, cell size and cell cycle

Human colon adenocarcinoma cells (WiDr) and Chinese hamster lung fibroblasts cells (V79) were incubated with different concentrations of 5-aminolevulinic acid (ALA), and the production of protoporphyrin IX (PpIX) was studied using several techniques. The amount of PpIX produced per cell increased with increasing ALA concentration according to different kinetics for the 2 cell lines. For both cell lines a cell density dependency of the PpIX synthesis was observed. For saturating ALA concentrations, 2-3 times more PpIX was produced per cell at a density of 5 x 10(4) than at a density of 5 x 10(3) cells/cm2. The photosensitivity of cells appeared to increase even more than the PpIX content, indicating a cooperative effect in inactivation. The PpIX production rate increased with cell size and was about 1.9 times higher for cells in the G2 + M phase than for cells in the G1 phase of the cell cycle. Neither cell size nor cell cycle distribution were significantly dependent on cell density.

Photosensitization of experimental hepatocellular carcinoma with protoporphyrin synthesized from administered delta-aminolevulinic acid: studies with cultured cells and implanted tumors

BACKGROUND/AIMS

Photodynamic therapy using porphyrins or related compounds and laser light is an investigational treatment for neoplasms. The aim of this study was to establish whether this might be applicable for hepatocellular carcinoma using protoporphyrin synthesized in the tissue from administered delta-aminolevulinic acid.

METHODS

We measured porphyrin accumulation in normal rat hepatocytes and Morris hepatoma cells in culture, and in subcutaneously implanted hepatomas and other tissues of the rat after administration of delta-aminolevulinic acid, and assessed cell and tissue damage after application of laser light.

RESULTS

Porphyrin accumulation after delta-aminolevulinic acid was added to the medium was greater and continued to increase for a longer period of time in hepatoma cells than in hepatocytes (1337+/-42 vs 513+/-31 fluorescence units/cell at 8 h, means+/-SE, p<0.001). After intraperitoneal injection of delta-aminolevulinic acid to rats with subcutaneously growing hepatomas, porphyrin content in tumor and liver was similar at 4 h but was higher in tumor at 6 h. Laser light caused necrosis of normal and malignant liver cells in culture and subcutaneous hepatomas in vivo.

CONCLUSIONS

We conclude from these in vitro and in vivo studies that porphyrin accumulation after administration of delta-aminolevulinic acid in this hepatoma is substantial and time dependent, and delivery of laser light locally can cause tumor photosensitization and necrosis.

Time-dependent intracellular accumulation of delta-aminolevulinic acid, induction of porphyrin synthesis and subsequent phototoxicity

Photodynamic therapy (PDT), a novel treatment for a variety of human malignancies, usually consists of visible light irradiation of lesions following the systemic administration of a photosensitizer. Induction of the endogenous photosensitizer protoporphyrin IX by the systemic or topical administration of delta-aminolevulinic acid (delta-ALA) is being investigated for use in PDT. We have determined that the incubation of two human and two rodent tumor cell lines in culture with delta-ALA over a 24 h period results in an increase in the accumulation of fluorescent porphyrins in all of these cell lines. However, the two human cell lines produce fluorescent porphyrin at different rates from those seen in the rodent cell lines. The uptake of 14C-delta-ALA was concentration dependent, similar for all the cell lines studied and rapidly reached an intra/extracellular equilibrium after delta-ALA was added to the culture medium. The increase in intracellular fluorescent porphyrin was dependent on the level of delta-ALA in the medium and the incubation time and was directly related to the phototoxicity observed upon exposure of cultured monolayers to light. The data demonstrate that equivalent levels of phototoxicity can be attained by exposing cells to 0.04 mM delta-ALA for 24 h or to 0.5 mM delta-ALA for 2 h. These findings may have implications for optimization of PDT treatment regimens that use delta-ALA.

Stimulation of tetrapyrrole synthesis in mammalian epithelial cells in culture by exposure to aminolaevulinic acid

Tetrapyrrole synthesis in CNCM-1221 cells exposed to 0.6 mM aminolaevulinic acid (ALA) was found to be approximately linear over a 6-h period of incubation. The rate was not significantly affected by cell density over a range of 0.015 to 0.15 x 10(6) cells cm(-2) (final cell density). Tetrapyrrole synthesis was not affected by GABA or glutamic acid in concentrations up to 6 mM and 2.72 mM respectively, suggesting that these amino acids, which are similar in structure to ALA, do not competitively inhibit the ALA uptake pathway in these cells. Pre-exposure to haem arginate (up to 100 microM) was inhibitory, presumably by suppression (through the inhibition of ALA synthase) of an endogenous component of the response. The ALA-stimulated response was not modified by co-exposure to AIA (up to 100 mg ml(-1)). Despite significant reduction of protein synthesis, the porphyrinogenic response of cells exposed to ALA was unaffected by cycloheximide (10 microg ml(-1)) or actinomycin D (10 microg ml(-1)) even when cells were preincubated with these agents for 3 h before ALA exposure. Fetal bovine serum (10%) inhibited tetrapyrrole synthesis by 30% but increased the rate of porphyrin export by cells by a factor of 1.5. The uptake of [14C]ALA was shown to be strongly influenced by the density of the cultures. In dense cultures (final cell density of approximately 0.15 x 10(6) cells cm(-2)), the ALA uptake rate was less than 0.8 compared with a maximum rate of 4.2 fmol per cell h(-1) at a cell density of 0.02 x 10(6) cells cm(-2). Since tetrapyrrole synthesis is less affected than ALA uptake by cell density, the resultant discrepancy in ALA incorporation occurring in dense cultures implies that endogenous ALA synthesis is induced in these cells. ALA uptake was not affected by cycloheximide or actinomycin D in serum-free conditions. However, fetal bovine serum decreased external ALA uptake by about 50%. This effect was abrogated by preincubation with cycloheximide.

Subcellular localization of Photofrin and aminolevulinic acid and photodynamic cross-resistance in vitro in radiation-induced fibrosarcoma cells sensitive or resistant to photofrin-mediated photodynamic therapy

The subcellular and, specifically, mitochondrial localization of the photodynamic sensitizers Photofrin and aminolevulinic acid (ALA)-induced protoporphyrin-IX (PpIX) has been investigated in vitro in radiation-induced fibrosarcoma (RIF) tumor cells. Comparisons were made of parental RIF-1 cells and cells (RIF-8A) in which resistance to Photofrin-mediated photodynamic therapy (PDT) had been induced. The effect on the uptake kinetics of Photofrin of coincubation with one of the mitochondria-specific probes 10N-Nonyl acridine orange (NAO) or rhodamine-123 (Rh-123) and vice versa was examined. The subcellular colocalization of Photofrin and PpIX with Rh-123 was determined by double-label confocal fluorescence microscopy. Clonogenic cell survival after ALA-mediated PDT was determined in RIF-1 and RIF-8A cells to investigate cross-resistance with Photofrin-mediated PDT. At long (18 h) Photofrin incubation times, stronger colocalization of Photofrin and Rh-123 was seen in RIF-1 than in RIF-8A cells. Differences between RIF-1 and RIF-8A in the competitive mitochondrial binding of NAO or Rh-123 with Photofrin suggest that the inner mitochondrial membrane is a significant Photofrin binding site. The differences in this binding may account for the PDT resistance in RIF-8A cells. With ALA, the peak accumulations of PpIX occurred at 5 h for both cells, and followed a diffuse cytoplasmic distribution compared to mitochondrial localization at 1 h ALA incubation. There was rapid efflux of PpIX from both RIF-1 and RIF-8A. As with Photofrin, ALA-induced PpIX exhibited weaker mitochondrial localization in RIF-8A than in RIF-1 cells. Clonogenic survival demonstrated cross-resistance to incubation in PpIX but not to ALA-induced PpIX, implying differences in mitochondrial localization and/or binding, depending on the source of the PpIX within the cells.

Factors affecting aminolaevulinic acid-induced generation of protoporphyrin IX

Photodynamic therapy (PDT) may cause tumour cell destruction by direct toxicity or by inducing cellular hypoxia as a result of microcirculatory shutdown. Aminolaevulinic acid (ALA) causes cellular accumulation of protoporphyrin IX (PPIX) in cells exposed to it in excess. PPIX can be used as a photosensitizer for PDT. Microcirculatory shutdown may be induced by toxicity to the endothelial and vascular smooth muscle (VSM) cells or by release of vasoactive substances. We have studied whether PPIX is produced by endothelial, VSM and tumour cells on exposure to ALA and whether these cell lines are directly damaged by PDT in vitro. Tumour endothelial cells are angiogenic and we have, therefore, investigated the effect of cellular proliferation rates on PPIX generation. Tumour cells generate more PPIX intracellularly than the non-neoplastic cell lines studied and are correspondingly more sensitive to PDT-induced cytotoxicity. Endothelial cells are sensitive to PDT-induced cytotoxicity and accumulate between 1.5 and four times more PPIX when proliferating (as during tumour-induced angiogenesis) than when quiescent. We conclude that PPIX-mediated PDT may exert some of its effects on the microcirculation of treated tissues by direct toxicity to endothelial and VSM cells, and that this toxicity may be enhanced in the tumour microenvironment.

In-vitro investigation of ALA-induced protoporphyrin IX

Protoporphyrin IX (PpIX) induced endogenously by delta-aminolevulinic acid (ALA), can be used to destroy photodynamically tumor cells. The influence of several parameters on the PpIX formation of human skin fibroblasts was investigated by fluorescence spectrophotometry. The PpIX formation increases (1) with the pH value of ALA (2) with the ALA incubation time in a moderate sigmoidal manner, and (3) with the ALA concentration up to 700 micrograms ml-1. Other parameters, such as cell washing procedures, have no influence on the PpIX production. ALA has to be applied in a concentration 30 times higher than external protoporphyrin IX and Photosan 3 in order to produce the same cytotoxic damage. Protoporphyrin bleaching and photoproduct generation at 646 nm was observed. Additional information about intracellular PpIX formation kinetics and its topographically correlation to cell structures was gained by a CCD camera mounted on a fluorescence microscope. A few minutes after the onset of incubation with ALA, PpIX generation is observed in the mitochondria, followed by relocalization in the cytoplasm and the nuclear membrane.

The effect of brief illumination on intracellular free calcium concentration in cells with 5-aminolevulinic acid-induced protoporphyrin IX synthesis

The effect of illumination on intracellular free calcium concentration, [Ca2+]i, was studied in a cell line (WiDr cells) derived from a primary adenocarcinoma of the rectosigmoid colon. In these cells the biosynthesis of protoporphyrin IX was stimulated by 5-aminolevulinic acid to reach levels of 600-700 pmol of protoporphyrin IX per mg cell protein. A brief (1-min) exposure of the cells to light (70% of light energy at 340-380 nm) resulted in an increase in [Ca2+]i. This increase was not reversible over a period of at least 20 min following illumination. Elevation of [Ca2+]i most probably represented an influx of calcium ions from the medium to the cell, since it was completely abolished in the presence of extracellular EGTA. The increased [Ca2+]i did not reflect general membrane damage, as determined by trypan blue staining as well as measurement of the intercalation of ethidium bromide into cellular DNA, and neither did the sustained elevation of [Ca2+]i lead to any substantial loss of clonogenicity following illumination of protoporphyrin-containing cells. Together these results indicate that an increased [Ca2+]i level is not per se a cause of cell death during photodynamic therapy.

Aminolevulinic acid for photodynamic therapy of bladder carcinoma cells

A new concept in photosensitizing tumor cells is photosensitizer synthesis in situ. Aminolevulinic acid (ALA) is a precursor of protoporphyrin IX (PP IX), a potent photosensitizer. The goal of our study was to examine dark toxicity, phototoxic potential, metabolism of ALA and morphological alterations in Waf bladder cancer cells. Dark toxicity of Waf cells was observed after incubation with ALA, beginning at a concentration of 15 mM. Photodynamic treatment with ALA at concentrations of 1, 5 and 10 mM showed a drug- and light-dose-dependent cell survival rate in comparison to a control group. Two incubation times of 3.5 and 5.5 h were compared for cell survival. After a longer incubation time of 5.5 h, cell survival was decreased in all experiments; this is consistent with our extraction data where higher fluorescence was found after 5.5 than after 3.5 h. The results show that ALA-induced photosensitization has a high potential for photodynamic therapy (PDT) of superficial bladder carcinoma.

Photodynamic effects of 5-aminolevulinic acid-induced porphyrin on human bladder carcinoma cells in vitro

The efficiency of 5-aminolevulinic acid (ALA) in photodynamic therapy (PDT) was investigated in vitro using urothelial carcinoma cells of various differentiation. HCV29, RT4 and J82 cells were cultured in 96-well plates, incubated with 25-100 micrograms/ml ALA in serum-containing medium for 4 h, and irradiated at 630, 635 and 640 nm wavelength with light doses of 15-100 J/cm2. The degree of reduced tetrazolium bromide corresponding to cell viability was determined with a colorimetric MTT assay 0, 24 and 48 h after PDT. A remarkable reduction of mitochondrial activity occurred in poorly (J82) and well differentiated (RT4) malignant urothelial cells. Twenty-four hours after photodynamic treatment with 100 micrograms/ml ALA and 50 J/cm2, the metabolic activity of malignant cells was nearly extinguished, while HCV29 cells, derived from normal urothelium, behaved similarly to non-irradiated control cells. The photosensitivity of cells depended on presence or absence of fetal bovine serum (FBS) in the ALA-incubation medium. A wavelength of 635 nm was up to 60% more effective compared with 630 nm, which is more frequently applied in PDT. From the results of our in vitro studies, we can define a "therapeutic window" for malignant cells without damaging benign cells. The time delayed effects and the strong wavelength dependence are important factors for a clinical application.

Subcellular damage kinetics within co-cultivated WI38 and VA13-transformed WI38 human fibroblasts following 5-aminolevulinic acid-induced protoporphyrin IX formation

The generation of the photosensitizer protoporphyrin IX (PpIX) in cells can be induced by externally applied 5-aminolevulinic acid (ALA), with that bypassing the feedback control mechanism. The aim of the present study was to investigate the onset of destructive changes in living cocultivated WI38 and VA13-transformed WI38 human fibroblasts following ALA incubation, PpIX production and subsequent irradiation by white halogen light with a dose of 2.2 kJ/m2. Specific fluorescence markers such as 3,3'-dihexyloxacarbocyanine iodide for endoplasmic reticulum (ER) staining and dihydrorhodamine for intact mitochondria mapping combined with a low light imaging system are a versatile and sensitive tool to examine the photoinduced destruction of organelles in living cells, while artifacts are minimized. Mitochondria as primary targets of PpIX undergo a condensation under irradiation and are finally destroyed. Photodynamic treatment induces further a significant decomposition of ER, although PpIX localization could not be determined. Initial destabilization and vesiculation of ER is followed by a porous network with large cisternae (indicating the breakdown of cell integrity and cell/nucleus membrane damage). Normal cocultivated lung fibroblasts showed a delay in destruction compared to the transformed WI38-VA13 cells. The observed decomposition pattern resembles the morphological pattern of apoptosis.

Cellular fluorescence of the endogenous photosensitizer protoporphyrin IX following exposure to 5-aminolevulinic acid

Supplying 5-aminolevulinic acid (ALA), a precursor in the biosynthetic pathway to heme from an external source leads to an accumulation of the endogenous fluorescent photosensitizer protoporphyrin IX (PPIX). Following instillation of ALA in the urinary bladder neoplastic tissue can be discerned by fluorescence cystoscopy or treated by illumination with light of an appropriate wavelength. In order to provide a biological rationale for the clinical findings, we have analyzed the capacity of three different cell lines to accumulate PPIX by flow cytometry. Three different urothelial cell lines, normal fibroblasts and endothelial cells were exposed to ALA under varying conditions. Urothelial cell lines J82 and RT4, derived from malignancies of the bladder displayed fluorescence intensities 9- and 16-fold, respectively, above the fluorescence level of the normal urothelial cell line HCV29. Human umbilical cord endothelial cells fluoresced moderately while the fibroblast cell line N1 exhibited a fluorescence level comparable to those of the cancer cells. Fluorescence increased with increasing cell density and was also dependent on the growth of cells as monolayers or multicellular spheroids. Increasing ALA concentrations led to saturation of fluorescence after 4 h of incubation at cell type-specific fluorescence levels obtained at different ALA concentrations. Continuous incubation in medium containing serum resulted in a linear rise of fluorescence during the first 4 h, which was followed by a saturation period (8-24 h) and a renewed rise. In the case of serum depletion, fluorescence intensities were significantly higher and increased linearly during the entire 48 h incubation period.(ABSTRACT TRUNCATED AT 250 WORDS)

A mechanistic study of cellular photodestruction with 5-aminolaevulinic acid-induced porphyrin

5-Aminolaevulinic acid (ALA)-induced porphyrin biosynthesis and phototoxicity in vitro was investigated in five malignant and two normal cell lines. Intracellular protoporphyrin IX (PpIX) content was quantified by extraction and fluorescence spectroscopy. Cellular PpIX content did not always correlate with cell proliferation rate as measured by the doubling times of cell lines. Cellular efflux of PpIX was also investigated. In a bladder carcinoma cell line, the observed rapid efflux was not blocked by verapamil, an inhibitor of the P-glycoprotein efflux pump. These data support the view that cellular PpIX accumulation is a dynamic process that is determined by both the efflux of PpIX from the cells and enzyme activities in the haem biosynthesis pathway. Desferrioxamine (desferal), a modulator of PpIX biosynthesis, enhanced ALA-induced cellular PpIX content significantly in all carcinoma cell lines but not in non-malignant cell lines. The enhanced PpIX cellular accumulation is attributed to inhibition of ferrochelatase activity, the enzyme responsible for the conversion of PpIX to haem. PpIX-mediated cellular photodestruction following irradiation with an argon ion laser at 514.5 nm was determined by the 'MTT assay'. There appeared to be a 'threshold' effect of cellular PpIX content; cells that synthesised less than 140 ng/mg-1 protein exhibited very little phototoxic damage, while cell lines having greater than 140 ng PpIX/mg-1 protein [corrected] exhibited a consistent phototoxic response. Among the cell lines which did undergo phototoxic damage, there was not a strict correlation between PpIX cellular content and ALA-induced phototoxicity. Desferal enhanced the PpIX content and phototoxic effect in the responsive cells. Fluorescence microscopy of the ALA-treated cells revealed marked accumulation of PpIX in mitochondria (rhodamine 123 co-staining). That the primary site of phototoxic damage is also the mitochondria was confirmed by electron micrographs of cells photosensitised with ALA-induced PpIX, which showed swelling of mitochondria within minutes after irradiation while other suborganelles appeared to be unaffected. The repair or further destruction of the mitochondria was fluence and cell-type dependent. The data from this study suggest that the basis of increased ALA-induced PpIX accumulation in tumours is a combination of various aspects of the metabolic process and pharmacokinetics and that the efficacy of photodestruction of malignancy will be determined not only by the rate of PpIX synthesis but also by specific cellular and tissue characteristics.

Porphyrins, porphyrias, cancer and photodynamic therapy--a model for carcinogenesis

Porphyrins are the only and most powerful photosensitizers synthesized internally. To understand better the involvement of porphyrins in photosensitization reactions, the heme biosynthetic pathway is first described, as well as the main features of its regulation in both erythroid and hepatic cells. Most disorders of porphyrin metabolism, known as porphyrias, are characterized by porphyrin accumulation. A full discussion of these diseases, their classification and relevant biochemical and clinical signs are presented. Abnormalities in heme biosynthesis in disorders other than porphyrias, such as iron-deficient and sideroblastic anemias, lead poisoning, hereditary tyrosinemia, chronic renal disease and alcoholism, are briefly considered. A complete survey of the experimental research on the biosynthesis of porphyrins in tumors and of the important association between cancer and porphyrias is dealt with. The link to photodynamic therapy (PDT) emerges naturally and this is treated from the point of view of using porphyrins endogenously formed by the tumors for their localization and PDT. Finally, considering the nature of the alterations occurring in heme metabolism in tumors, and porphyrias and their ubiquity, a model is discussed where the abnormality of heme synthesis is involved in the initiating lesion of carcinogenesis. The model strongly predicts that the incidence of cancer will be high in cells with abnormal heme metabolism, suggesting that porphyric patients may be at greater risk of the development of cancer.