Intracellular glutathione and cytotoxicity of platinum complexes

A Potent Strategy of Combinational Blow Toward Enhanced Cancer Chemo-Photodynamic Therapy via Sustainable GSH Elimination

Excessive glutathione (GSH), which is produced owing to abnormal metabolism of tumor cells, scavenges photo-induced reactive oxygen species (ROS) and consumes chemotherapeutic drugs, thereby attenuating the efficacy of photodynamic therapy and chemotherapy, respectively. Predominant strategies for GSH inhibition involve its chemical depletion, which only leads to a temporary therapeutic effect because GSH is replenished via various compensatory routes in tumor cells. Here, a versatile GSH-inhibiting nanosystem (termed PCNPs) for persistent synergistic therapy of cancer is reported. The porous skeleton of PCNPs allows easy encapsulation of buthionine sulfoximine (BSO) to sustainably suppress the biosynthesis of GSH. Thus, PCNPs not only demonstrate a prolonged release of BSO and improve drug utilization for efficient chemotherapy, but also act as an efficient photo-induced singlet oxygen radical generator that prevents the loss of ROS, thereby enhancing photodynamic therapy. In addition, the liposomal coating prevents cargo release in the blood, improves the accumulation of PCNPs at the tumor site, and promotes the cellular uptake of oxaliplatin and BSO. This strategy is applicable to ROS-based therapy and chemotherapy, which are suppressed by GSH, and may further enhance the synergistic effect of GSH-restrained therapy.

Through oxaliplatin resistance induction in colorectal cancer cells, increasing ABCB1 level accompanies decreasing level of miR-302c-5p, miR-3664-5p and miR-129-5p

Oxaliplatin as a component of (Neo-) adjuvant chemotherapeutic regimens is administered to colorectal cancer patients. Unfortunately, the acquisition of resistance to this drug in nearly 90% of metastatic patients rendered it as an ineffective drug. Therefore, resistance mechanisms to this drug should be elucidated. There are different genes like GSTP1 and ABCB1 which are responsible for oxaliplatin resistance. We hypothesized that miR-129-5p, miR-302c-5p, miR-3664-5p, mir-3714 and miR-513a-3p are targeting ABCB1 gene, while GSTP1 was predicted to be the potential target of miR-3664-5p, mir-3714 and miR-513a-3p. In order to study this hypothesis, resistant colorectal cell lines were generated through intermittent exposure of HCT116, SW480 and HT29 to the increasing doses of oxaliplatin. MTT assays validated this resistance induction. Expression of ABCB1 and GSTP1 in addition to their targeting miRNAs in different cell lines were studied by quantitative real time PCR in the cell lines. Even though in comparison with HCT116 and SW480 cell lines, GSTP1 expression was reduced in resistant cells, ABCB1 expression was upregulated in these cell lines. On the other hand, HT-29 resistant cells showed elevated GSTP1 and unchanged ABCB1 levels. While miR-302c-5p level was downregulated in resistant cell lines, miR-129-5p and miR-3664-5p level showed different pattern of reduction in the resistant SW480 and HCT116 cell lines. GSTP1 level was correlated directly with miR-513a-3p and miR-3664-5p in all SW480 and HCT116 derived cell lines, however in HT-29-OXR1, GSTP1 level was correlated inversely with miR-3664-5p. In conclusion, upregulation of ABCB1 can be considered as the crucial component of poor response to oxaliplatin which is likely controlled by miR-302c-5p.

Tumor-Related Molecular Mechanisms of Oxaliplatin Resistance

Oxaliplatin was the first platinum drug with proven activity against colorectal tumors, becoming a standard in the management of this malignancy. It is also considered for the treatment of pancreatic and gastric cancers. However, a major reason for treatment failure still is the existence of tumor intrinsic or acquired resistance. Consequently, it is important to understand the molecular mechanisms underlying the appearance of this phenomenon to find ways of circumventing it and to improve and optimize treatments. This review will be focused on recent discoveries about oxaliplatin tumor-related resistance mechanisms, including alterations in transport, detoxification, DNA damage response and repair, cell death (apoptotic and nonapoptotic), and epigenetic mechanisms.

Literature review and practical aspects on the management of oxaliplatin-associated toxicity

Colorectal cancer is currently a public health priority because it is the second leading cause of cancer deaths in Western countries. Combination regimes of oxaliplatin and infusional fluorouracil/leucovorin or capecitabine have emerged as important options in the palliative and adjuvant treatment of colorectal cancer. Although better tolerated than cisplatin, oxaliplatin displays a characteristic profile of adverse events whose recognition and management are essential for physicians who treat patients with colorectal cancer and other malignancies that benefit from the use of oxaliplatin. Peripheral neuropathy is probably the most frequent and clinically relevant adverse event associated with the use of oxaliplatin, and several measures have been proposed to mitigate this toxicity. Temporary interruption of oxaliplatin before limiting neurotoxicity develops during therapy is a potential approach to avoid the problem of oxaliplatin-associated neuropathy in patients with metastatic colorectal cancer. Calcium and magnesium infusions have no effect on chemotherapy efficacy and also constitute a useful approach in clinical practice. Finally, the incidence and severity of chronic peripheral neuropathy in patients treated with oxaliplatin may be reduced by the use of neuroprotective agents, for example, venlafaxine. Other adverse events, such as gastrointestinal and liver toxicity, thrombocytopenia, and hypersensitivity reactions, are also reviewed in this article, and suggestions are made for their management.

Anticancer activity of metal complexes: involvement of redox processes

Cells require tight regulation of the intracellular redox balance and consequently of reactive oxygen species for proper redox signaling and maintenance of metal (e.g., of iron and copper) homeostasis. In several diseases, including cancer, this balance is disturbed. Therefore, anticancer drugs targeting the redox systems, for example, glutathione and thioredoxin, have entered focus of interest. Anticancer metal complexes (platinum, gold, arsenic, ruthenium, rhodium, copper, vanadium, cobalt, manganese, gadolinium, and molybdenum) have been shown to strongly interact with or even disturb cellular redox homeostasis. In this context, especially the hypothesis of "activation by reduction" as well as the "hard and soft acids and bases" theory with respect to coordination of metal ions to cellular ligands represent important concepts to understand the molecular modes of action of anticancer metal drugs. The aim of this review is to highlight specific interactions of metal-based anticancer drugs with the cellular redox homeostasis and to explain this behavior by considering chemical properties of the respective anticancer metal complexes currently either in (pre)clinical development or in daily clinical routine in oncology.

A phase II trial and pharmacokinetic study of oxaliplatin in children with refractory solid tumors: a Children's Oncology Group study

BACKGROUND

Platinating agents are used in the treatment of a spectrum of childhood cancers. Oxaliplatin, a third generation platinum compound, may provide less toxicity and be more effective. A phase 2 study was performed to estimate the response rate to single agent oxaliplatin in patients with refractory pediatric solid tumors, and to further describe the toxicities and pharmacokinetics of the drug in this population.

PATIENTS AND METHODS

Subjects, < or =21 years of age at original diagnosis, received oxaliplatin (130 mg/m(2)) intravenously every 21 days. Prior platinum exposure was acceptable. Histologies included: Ewing sarcoma/peripheral PNET, osteosarcoma, rhabdomyosarcoma, neuroblastoma, high and low grade astrocytoma, brain stem glioma, ependymoma, hepatoblastoma and selected rare tumors. A two-stage design, enrolling 10 + 10 subjects, was used for each disease stratum. Limited sampling pharmacokinetic studies were performed.

RESULTS

Of 124 eligible subjects (75 males), 113 were evaluable for response and 69 (62%) had received platinum previously. Only one objective response was observed, a partial response in a 6-year-old child with ependymoma. An additional 13 subjects with various other solid tumors had stable disease, receiving a median (range) of 13.5 (2-17) cycles. Five subjects completed 17 treatment cycles. Thrombocytopenia was the most common toxicity observed. The median (range) terminal half-life and clearance for ultrafiltrable platinum were 293 (187-662 hr) and 14.0 (1.9-24.9 L/hr/m(2)), respectively (n = 49).

CONCLUSIONS

Although reasonably well tolerated, oxaliplatin administered as a single agent has limited activity in pediatric patients with relapsed or refractory solid tumors.

Glutathione-dependent interaction of heavy metal compounds with multidrug resistance proteins MRP1 and MRP2

The interactions of three heavy metal-containing compounds, cisplatin (CDDP), arsenic trioxide (As(2)O(3)), and mercury dichloride (HgCl(2)), with the multidrug resistance transporters MRP1 and MRP2 and the involvement of glutathione (GSH)-related processes herein were investigated. In Madin-Darby canine kidney cells stably expressing MRP1 or MRP2, viability, GSH content, calcein efflux and polarized GSH efflux were measured as a function of exposure to CDDP, As(2)O(3) and HgCl(2). In isolated Sf9-MRP1 and Sf9-MRP2 membrane vesicles, the interaction with MRP-associated ATPase activity was measured. In the latter model system adduct formation with GSH is not an issue. The data show that (1) CDDP interacts with both MRP1 and MRP2, and GSH appears to play no major role in this process, (2) As(2)O(3) interacts with MRP1 and MRP2 in which process GSH seems to be essential, and (3) HgCl(2) interacts with MRP1 and MRP2, either alone and/or as a metal-GSH complex.

Correlations between the activities of 19 anti-tumor agents and the intracellular glutathione concentrations in a panel of 14 human cancer cell lines: comparisons with the National Cancer Institute data

The aim of this work was 2-fold: (i) to identify correlations between the activities of pairs of 19 anti-tumor agents in a mini-panel of 14 human cancer cell lines of diverse origins with the goal of validating the panel, and (ii) to look for correlations between the activities of 19 standard anti-tumor agents and the intracellular concentrations of glutathione (GSH). Validation with analogous data from the National Cancer Institute (NCI) Developmental Therapeutics Program was made. The cell growth inhibition potencies of the anti-tumor agents [cisplatin, carboplatin, oxaliplatin, DACH-Pt, melphalan, chlorambucil, thiotepa, busulfan, doxorubicin, etoposide, camptothecin, vinblastine, podophyllotoxin, colchicine, taxol, hydroxyurea, methotrexate, 5-azacytidine and 5-fluorouracil (5-FU)] were estimated in 14 cancer cell lines by their GI50 values. An enzymatic assay based on the method of Tietze was employed to measure intracellular total GSH concentrations. The Delta method was used to compare pairs of anti-tumor agents; similarities and differences in activity profiles (mean graphs) were evaluated by regression analysis. Most, but not all, of the correlations could be explained based on similarities in the mechanisms of action and many correlations/non-correlations were also observed in the NCI data. Some correlations were unexpected however, and not seen in the NCI data. For example, strong positive correlations (P < 0.01) were found between the GI50 values of melphalan/chlorambucil and the anti-mitotic agents. Similarly unexpected, a strong positive correlation was observed between methotrexate and cisplatin (P < 0.01). Interestingly, moderate to strong negative correlations (P < 0.01-0.05) were found between the GI50 values of 5-FU and the anti-mitotic agents/melphalan/chlorambucil. Significant positive correlations between intracellular GSH concentrations and GI50 values were found only for thiotepa (P < 0.05) and doxorubicin (P < 0.01). Data from a NCI panel of 34 cancer cell lines showed no correlations between GSH levels and the GI50 values of the same 19 compounds. In conclusion, a panel of 14 human cancer cell lines of diverse origin was used to identify similarities and differences in the activities of standard anti-tumor agents. The level of significance was stronger with the 34 cell lines of the NCI, however. Our results indicate that GSH intracellular concentrations correlate with resistance only with doxorubicin and thiotepa in these cell lines.

No correlation between GSH levels in human cancer cell lines and the cell growth inhibitory activities of platinum diamine complexes

Increases in the intracellular levels of glutathione (GSH) in cancer cells have been implicated in the development of acquired resistance to platinum antitumor agents. On the other hand, little information is available on the relationships between intracellular GSH levels in non-treated cancer cells and their response to platinum complexes. The present work investigated for possible correlations between concentrations of intracellular GSH/GSSG in 14 human cancer cell lines growing in vitro and the cell growth inhibitory activities of cisplatin, carboplatin, oxaliplatin, and d,l-trans-1,2-diaminocyclohexane-dichloro-platinum(II) (DACH-Pt). No statistically significant correlation between GSH levels and the activities of any of the four Pt-complexes could be found.

Kinetic Study on the Reactions of Platinum Drugs with Glutathione

The binding of platinum (Pt) drugs (oxaliplatin, carboplatin, and cisplatin) to glutathione (GSH, 6.75 mM) was investigated at 37 degrees C in Hepes (100 mM, pH approximately 7.4) or Tris-NO(3) (60 mM, pH 7.4) buffer and NaCl (4.62, 6.63, or 7.82 mM). The conditions were chosen to mimic passage of clinical concentrations of the drugs (135 microM) through the cytosol. The reactions were monitored by UV-absorption spectroscopy, high-performance liquid chromatography (HPLC), and atomic absorption spectroscopy. The initial rates, detected by UV absorbance, were similar for oxaliplatin and cisplatin reacting with GSH and were more than 5-fold faster than for carboplatin reacting with GSH. The Pt contents in HPLC eluates corresponding to unbound drug decreased exponentially with time, confirming that the reactions were first order in [Pt drug] and allowing determination of the pseudo first-order rate constants (k(1)). The second-order rate constants (k(2)) were calculated as k(1) divided by [GSH]. The k(2) value for oxaliplatin reacting with GSH was approximately 3.8 x 10(-2) M(-1) s(-1), for cisplatin reacting with GSH approximately 2.7 x 10(-2) M(-1) s(-1), and for carboplatin reacting with GSH approximately 1.2 x 10(-3) M(-1) s(-1) (approximately 32-fold slower than that of oxaliplatin and approximately 23-fold slower than that of cisplatin). These results demonstrate an influence of ligands surrounding the Pt coordination sphere on the reactivity of Pt(2+) with GSH.

Nicotinamide adenine dinucleotide phosphate-regenerating system coupled to a glutathione-reductase microtiter method for determination of total glutathione concentrations in adherent growing cancer cell lines

Improvements in the traditional glutathione (GSH)-reductase recycling method for determining total glutathione levels in adherent growing cells have been achieved by eliminating the direct use of expensive nicotinamide adenine dinucleotide phosphate (NADPH) and normalizing the levels of GSH to moles/liter instead of the more usual but more error-prone method of normalizing with cellular protein. A glucose-6-phosphate-dehydrogenase auxiliary reaction has been added to the microtiter-adapted enzyme method of Tietze; thus NADP(+) and glucose-6-phosphate replace NADPH in the method. This modification lowers the possibility for substrate inhibition of the reductase by high levels of NADPH during the initial phase of the reaction while at the same time reducing the assay costs by 75-85%. To calculate the cellular concentration of GSH, the number of cells used for the GSH determination, estimated by counting cell nuclei of benzalkonium chloride-lysed cells with a Coulter Counter Z2, and the average cell volume, also determined with the Coulter Counter, are multiplied to give the total sample volume. The quotient of the amount of GSH found in the cells and the total sample volume yields the GSH concentration in moles/liter. The assay has been validated with respect to precision (+/-2.6%), relative accuracy (-4.2 %), linearity (r(2)=0.98), linear range (0.5-10 microM), and limit of detection (80 pmol). Recovery was cell line dependent and ranged between 70 and 103% in the six cell lines. As an application of this method, the GSH concentrations in six human cancer cell lines were determined, without and with a 24-h preincubation with 200 microM D,L-buthionine-S,R-sulfoximine (BSO), an inhibitor of GSH biosynthesis. As expected, BSO lowered GSH levels on the average 85%.

Platinum-based anticancer agents: innovative design strategies and biological perspectives

The impact of cisplatin on cancer chemotherapy cannot be denied. Over the past 20 years, much effort has been dedicated to discover new platinum-based anticancer agents that are superior to cisplatin or its analogue, carboplatin. Most structural modifications are based on changing one or both of the ligand types coordinated to platinum. Altering the leaving group can influence tissue and intracellular distribution of the drug, whereas the carrier ligand usually determines the structure of adducts formed with DNA. DNA-Pt adducts produced by cisplatin and many of its classical analogues are almost identical, and would explain their similar patterns of tumor sensitivity and susceptibility to resistance. Recently some highly innovative design strategies have emerged, aimed at overcoming platinum resistance and/or to introduce novel mechanisms of antitumor action. Platinum compounds bearing the 1,2-diaminocyclohexane carrier ligand; and those of multinuclear Pt complexes giving rise to radically different DNA-Pt adducts, have resulted in novel anticancer agents capable of circumventing cisplatin resistance. Other strategies have focused on integrating biologically active ligands with platinum moieties intended to selectively localizing the anticancer properties. With the rapid advance in molecular biology, combined with innovation, it is possible new Pt-based anticancer agents will materialize in the near future.

Cellular determinants of oxaliplatin sensitivity in colon cancer cell lines

Oxaliplatin (L-OHP) is a new platinum analogue that has shown antitumour activity against colon cancer both in vitro and in vivo and is now used in the chemotherapeutic treatment of metastatic colon and rectal cancer. L-OHP like cisplatin (CDDP), is detoxified by glutathione (GSH)-related enzymes and forms platinum (Pt)-DNA adducts lesions that are repaired by the nucleotide excision repair system (NER). We investigated the cytotoxicity and the pharmacology of L-OHP and CDDP on a panel of six colon cell lines in vitro. We showed that GSH and glutathione S-transferase (GST) activity were not correlated to oxaliplatin cytotoxicity. Pt-DNA adducts formation and repair were correlated with CDDP, but not with L-OHP cytotoxicity. The determination of ERCC1 and XPA expression, two enzymes of the NER pathway, by reverse transcriptase-polymerase chain reaction (RT-PCR), demonstrated that ERCC1 expression was predictive of L-OHP sensitivity (r(2)=0.67, P=0.02) and XPA level after oxaliplatin exposure was also correlated to L-OHP IC(50) (r(2)=0.5; P=0.04). The knowledge of such correlations could help predict the sensitivity of patients with colon cancer to L-OHP.

Glutathione induces cellular resistance against cationic dinuclear platinum anticancer drugs

The sulfur-containing tripeptide glutathione (GSH) is one of the most abundant molecules in cells. Elevated levels of GSH render some types of cancer cells resistant against well-known platinum anti-cancer drugs such as cisplatin and carboplatin. Platinum complexes are often very reactive towards the cysteine residue of GSH, which detoxifies these compounds by a rapid binding mechanism. Clearly, this resistance mechanism poses a severe obstacle to any new platinum drugs designed to overcome cisplatin resistance. In the present study the cytotoxicity of dinuclear platinum compounds of the 1,1/t,t type, as developed by Farrell, is determined in human ovarium A2780 cells and in the cisplatin-resistant cell line A2780cisR, which possesses elevated levels of GSH. Further, the effect of depletion of GSH levels by L-buthionine-S,R-sulfoximine (L-BSO) in A2780cisR was investigated. The experiments show that detoxification by GSH is an effective resistance mechanism against dinuclear platinum compounds. However, the dinuclear complexes are less sensitive towards detoxification compared to cisplatin. This is probably because of the rapid binding of dinuclear cationic complexes to DNA. Compared to cisplatin, the rapid binding to DNA reduces the time during which the drug molecules are exposed to GSH in the cytosol. The reaction of a representative dinuclear compound with glutathione (pH 7, 37 degrees C) was studied in detail by 195Pt NMR. The dinuclear complex BBR3005 ([trans-PtCl(2)(NH(3))(2)(mu-H(2)N(CH(2))(6)NH(2))](2+), abbreviated as 1,1/t,t n=6), follows different pathways in the reaction with GSH, depending on the molar ratio of the reactants. When reacted in stoichiometric amounts (1:1), first a chloride on each platinum is replaced by a sulfur, forming a PtN(3)S product at -2977 ppm. After 2-3 h, this intermediate reacts further to form a sulfur-bridged N(3)Pt-S-PtN(3) species as the main product at -2811 ppm. When BBR3005 is reacted with GSH in a ratio of 1:4, the sulfur-bridged species is not observed. Instead, the final product is trans-Pt(GS)(2)(NH(3))(2) (at -3215 ppm); the same product appears if GSH is reacted with trans-PtCl(2)(NH(3))(2). Apparently, GSH first replaces the chlorides and subsequently degrades the dinuclear compound by replacement of the diaminealkyl linker.

DNA interstrand cross-linking efficiency and cytotoxic activity of novel cadmium(II)-thiocarbodiazone complexes

We have prepared mono- and binuclear complexes of Zn(II) and Cd(II) with bis(2-pyridyl aldehyde) thiocarbodiazone (H(2)L(1)) and bis(methyl 3-pyridyl ketone) thiocarbodiazone (H(2)L(2)). Cytotoxicity data against the ovarian tumor cell line A2780cisR (acquired resistance to cisplatin) indicate that the mononuclear complex Cd/H(2)L(2) (1) and the binuclear complex Cd(2)/H(2)L(1) (4) are able to circumvent cisplatin resistance and that their cytotoxic activity does not substantially vary after depletion of intracellular levels of glutathione. Moreover, DNA binding studies show that complexes 1 and 4 have higher efficiency than cisplatin at forming DNA interstrand cross-links in both naked pBR322 plasmid and A2780cisR cellular DNA. Interestingly, the thiocarbodiazone ligands alone do not show the biological properties of complexes 1 and 4. Altogether these results suggest that DNA interstrand cross-link formation by compounds 1 and 4 might be related with their cytotoxic activity in cisplatin-resistant cells. We think that compounds 1 and 4 may represent a novel structural lead for the development of cadmium cytotoxic agents capable of improving antitumor activity in cisplatin-resistant tumors.

Oxaliplatin. A review of its pharmacological properties and clinical efficacy in metastatic colorectal cancer and its potential in other malignancies

Oxaliplatin is a platinum compound that inhibits DNA synthesis, primarily by causing intrastrand cross-links in DNA. Oxaliplatin has a broad spectrum of antineoplastic activity and has demonstrated a lack of cross-resistance with other platinum compounds. In patients with metastatic colorectal cancer, intravenous oxaliplatin has been trialled as a monotherapy and in combination with other agents. The highest response rates were achieved when oxaliplatin was used in combination with fluorouracil/folinic acid (leucovorin; calcium folinate), typically > or = 50% in the first-line setting and 13 to 45% as a second-line therapy. First-line triple therapy with oxaliplatin and fuorouracil/folinic acid achieved significantly higher response rates and longer median progression-free survival than fluorouracil/folinic acid therapy alone. However, no significant difference in the median duration of overall survival was found. This may be a consequence of the subsequent use of oxaliplatin and/or surgery after disease progression in patients who relapsed after fluorouracil/folinic acid therapy alone. Neoadjuvant therapy with oxaliplatin/fluorouracil/folinic acid has proven beneficial in enabling surgical removal of previously unresectable liver metastases. In 2 studies, surgery with curative intent was performed in 16 and 51% of patients with initially unresectable liver metastases following oxaliplatin/fluorouracil/folinic acid therapy; the 5-year survival rates were 40 and 50%, respectively. In patients with advanced ovarian cancer, first-line therapy with oxaliplatin/cyclophosphamide achieved an objective response rate which did not differ significantly from that of cisplatin/cyclophosphamide (33 vs 42%). In addition, oxaliplatin has shown efficacy in patients with platinum-pretreated ovarian cancer and achieved objective response rates similar to paclitaxel in this setting (16 vs 17%). Promising results have also been found with oxaliplatin in patients with non-Hodgkin's lymphoma, breast cancer, mesothelioma and non-small cell lung cancer. Reversible, cumulative, peripheral sensory neuropathy is the principle dose-limiting factor of oxaliplatin therapy. Haematological and gastrointestinal toxicities occur frequently but are generally mild to moderate in intensity.

CONCLUSION

Oxaliplatin in combination with fluorouracil/folinic acid is an effective treatment option for patients with metastatic colorectal cancer, both as a first-line therapy and in patients refractory to previous chemotherapy. Although preliminary results failed to show any overall survival advantage of this regimen over fluorouracil/folinic acid alone, this may be a consequence of trial design and requires further examination. Additional clinical investigation of oxaliplatin in patients with other cancers is warranted given the promising results achieved in early trials, most notably in patients with platinum-pretreated ovarian cancer.

Oxaliplatin: pharmacokinetics and chronopharmacological aspects

Oxaliplatin is the first clinically available diaminocyclohexane platinum coordination complex. The drug is non-cross-resistant with cisplatin or carboplatin and is one of the few active drugs against human colorectal cancer. Its cytotoxicity is synergistic with fluorouracil and folinic acid (leucovorin), the reference treatment for this disease. The main cumulative dose-limiting toxicity of oxaliplatin is peripheral sensory neuropathy. The drug can also produce diarrhoea, vomiting and haematological suppression. Unlike cisplatin, no renal failure or peripheral motor neuropathy have been reported and the sensory neuropathy is partly reversible. Unlike carboplatin, oxaliplatin produces only mild to moderate haematological toxicity. Oxaliplatin undergoes biotransformation into aquated forms in the blood, where 3 species can be found: total platinum, ultrafilterable or 'free' platinum and erythrocyte platinum. Flameless atomic absorption (FAAS) is used for assaying platinum concentration in various tissues. Inductively-coupled plasma mass spectrometry (ICP-MS), with a >10-fold lower sensitivity threshold than FAAS, was also used for the determination of oxaliplatin pharmacokinetics. The pharmacokinetics of oxaliplatin are described by a 3-compartment model. The drug rapidly crosses the cellular membrane as a result of its lipophilicity. Hence, at the end of a 2-hour infusion, approximately 40% of the blood platinum is found in erythrocytes. The distribution half-life of ultrafiltrated plasma platinum ranges from 10 to 25 minutes and its terminal elimination half-life is 26 hours (determined with FAAS) or 270 hours (ICP-MS). The elimination half-life of erythrocytic platinum is 12 to 50 days, close to that of erythrocytes. 30 to 50% of the platinum is recovered in the urine within 2 to 5 days, with renal clearance accounting for half of the total clearance of ultrafiltrated platinum. The total clearance of this species is correlated with the glomerular filtration rate. No pharmacokinetic-pharmacodynamic relationship has been established for oxaliplatin. Pharmacokinetic alterations produced by fluorouracil + folinic acid or irinotecan were minimal if any. The prolonged stability of oxaliplatin makes it suitable for continuous infusions over 4 to 5 days, with a delivery rate which can be either constant or chronomodulated (peak rate at 1600h), using programmable ambulatory pumps. Chronomodulation significantly reduces toxicity and improves antitumour activity as compared with constant rate infusion. These differences in pharmacodynamic properties were paralleled by differences in plasma concentration time courses. The different drug concentration profiles achieved with different infusional modalities may be useful tools for understanding the relationship between the pharmacokinetics and pharmacodynamics of oxaliplatin and may lead to further optimisation of its administration schedule and its combination with other drugs.

Oxaliplatin: a review of preclinical and clinical studies

Of the new generation platinum compounds that have been evaluated, those with the 1,2-diaminocyclohexane carrier ligand-including oxaliplatin--have been focused upon in recent years. Molecular biology studies and the National Cancer Institute in vitro cytotoxic screening showed that diaminocyclohexane platinums such as oxaliplatin belong to a distinct cytotoxic family, differing from cisplatin and carboplatin, with specific intracellular target(s), mechanism(s) of action and/or mechanism(s) of resistance. In phase I trials, the dose-limiting toxicity of oxaliplatin was characterized by transient acute dysesthesias and cumulative distal neurotoxicity, which was reversible within a few months after treatment discontinuation. Moreover, oxaliplatin did not display any, auditory, renal and hematologic dose-limiting toxicity at the recommended dose of 130 mg/m2 q three weeks or 85 mg/m2 q two weeks given as a two-hour i.v. infusion. Clinical phase II experiences on the antitumoral activity of oxaliplatin have been conducted in hundreds of patients with advanced colorectal cancers (ACRC). Single agent activity reported as objective response rate in ACRC patients is 10% and 20% overall in ACRC patients with 5-fluorouracil (5-FU) pretreated/refractory and previously untreated ACRC, respectively. Synergistic cytotoxic effects in preclinical studies with thymidylate synthase inhibitors, cisplatin/carboplatin and topoisomerase I inhibitors, and the absence of hematologic dose-limiting toxicity have made oxaliplatin an attractive compound for combinations. Phase II trials combining oxaliplatin with 5-FU and folinic acid ACRC patients previously treated/refractory to 5-FU showed overall response rates ranging from 21% to 58%, and survivals ranging from 12 to 17 months. In patients with previously untreated ACRC, combinations of oxaliplatin with 5-FU and folinic acid showed response rates ranging from 34% to 67% and median survivals ranging from 15 to 19 months. Two randomized trials totaling 620 previously untreated patients with ACRC, comparing 5-FU and folinic acid to the same regimen with oxaliplatin, have shown a 34% overall response rate in the oxaliplatin group versus 12% in the 5-FU/folinic acid group for the first trial; and 51.2% vs. 22.6% in the second one. These statistically significant differences were confirmed in time to progression advantage for the oxaliplatin arm (8.7 vs. 6.1 months, and 8.7 vs. 6.1 months, respectively). A small but consistent number of histological complete responses have been reported in patients with advanced colorectal cancer treated with the combination of oxaliplatin with 5-FU/folinic acid, and secondary metastasectomy is increasingly done by oncologists familiar with the combination. Based on preclinical and clinical reports showing additive or synergistic effects between oxaliplatin and several anticancer drugs including cisplatin, irinotecan, topotecan, and paclitaxel, clinical trials of combinations with other compounds have been performed or are still ongoing in tumor types in which oxaliplatin alone showed antitumoral activity such as ovarian, non-small-cell lung, breast cancer and non-Hodgkin lymphoma. Its single agent and combination therapy data in ovarian cancer confirm its non-cross resistance with cisplatin/carboplatin. While the role of oxaliplatin in medical oncology is yet to be fully defined, it appears to be an important new anticancer agent.

In vitro circumvention of cisplatin resistance by the novel sterically hindered platinum complex AMD473

A novel sterically hindered platinum complex, AMD473 [cis-aminedichloro(2-methylpyridine) platinum (II)], has been selected for phase I clinical trials due to commence in 1997. AMD473 was rationally designed to react preferentially with nucleic acids over sulphur ligands such as glutathione. This report documents the in vitro circumvention of acquired cisplatin resistance mechanisms in human ovarian carcinoma (HOC) cell lines by AMD473. In a panel of 11 HOC cell lines, AMD473 showed intermediate growth inhibition potency (mean IC50 of 8.1 microM) in comparison to cisplatin (mean IC50 of 2.6 microM) and carboplatin (mean IC50 of 20.3 microM). AMD473 showed only a 30.7-fold increase in IC50 value from the most sensitive to the most resistant HOC cell line, whereas for cisplatin it was 117.9-fold and for carboplatin 119.7-fold. AMD473 also showed significantly (P < 0.05) reduced cross-resistance to cisplatin in a panel of three cell lines with known acquired platinum drug resistance mechanisms (mean RF for AMD473 was 1.9, for cisplatin 9.1). Cellular accumulation of AMD473 was not reduced in two HOC cell lines (A2780cisR and 41McisR), in which reduced cisplatin accumulation is a major mechanism of acquired cisplatin resistance. AMD473 naked-DNA binding was significantly less affected (P < 0.05) than that of cisplatin by the presence of 5 mM glutathione. Also, AMD473 almost completely circumvented acquired cisplatin resistance in a cell line (A2780cisR) with fivefold elevated intracellular glutathione levels compared with the parent A2780 cell line when measured by clonogenic assay (RF 4.5 for AMD473 vs RF 18 for cisplatin). AMD473 also showed a lower increase in IC50 than cisplatin in an A2780 cell line model with artificially elevated glutathione levels. AMD473 DNA binding was slower than that of cisplatin on both naked and cellular DNA. AMD473 also formed DNA interstrand cross-links (ICLs) at a slower rate than cisplatin (peak ICL formation was at 5 h for cisplatin vs > or = 14 h for AMD473) after equitoxic doses were exposed to HOC cells for 2 h. AMD473 ICLs in the CH1 HOC cell line were slowly formed and showed no visible signs of being repaired 24 h after removal of drug. This was paralleled by a slower, longer lasting induction of p53 protein by equitoxic doses of AMD473 in HOC cell lines with wild-type p53. This new class of sterically hindered platinum compound, selected for clinical trial in 1997, may therefore elicit improved clinical response in intrinsically and acquired cisplatin-resistant tumours in the clinic.

Growth factors, glia and gliomas

The abilities of growth factors to cause normal cells to express the properties associated with transformed cells is discussed in specific reference to the oligodendrocyte-type-2 astrocyte (O-2A) progenitor cell. In the O-2A lineage, it has been possible to use growth factors and other defined molecules to induce or promote in normal cells all of the main properties of tumor cells, these being continued cell division in the absence of differentiation, more subtle modulations of self-renewal probabilities, promotion of cell migration and inhibition of programmed cell death. In addition to our studies on primary cells, our application to the growth of human tumor specimens of techniques utilized to study primary glial progenitor cells has allowed us to isolate a human glioblastoma multiforme (GBM)-derived population that expresses many properties otherwise uniquely expressed by oligodendrocyte-type-2 astrocyte (O-2A) progenitor cells. Hu-O-2A/Gb1 (for Human O-2A lineage Glioblastoma number 1) cells responded to similar mitogens and differentiation modulators as rodent O-2A progenitors, and generated cells with features of precursor cells, oligodendrocytes and astrocytes. Moreover, 1H-NMR analysis of amino acid composition demonstrated a striking conservation of types and quantities of free amino acids between the human tumour cells and the rodent primary cells. Hu-O-2A/Gb1 cells represent the first human glioma-derived population for which unambiguous lineage assignment has been possible. Our results thus demonstrate that the human O-2A lineage can contribute to one of the most malignant of glial tumours. Our analyses further indicate that at least two distinct glial lineages can generate glioblastomas. In addition, the highly diagnostic 1H-NMR spectrum expressed by Hu-O-2A/Gb1 cells raises the possibility of eventual non-invasive identification of tumors of this lineage.

Intracellular metabolism of the orally active platinum drug JM216: influence of glutathione levels

JM216 (bis-acetato ammine dichloro cyclohexylamine Pt IV) is an oral platinum complex presently undergoing phase II clinical trials. Previous studies have identified some of its biotransformation products in clinical materials. This study evaluated the nature of JM216 biotransformation products intracellularly in two different human ovarian carcinoma cell lines, one relatively sensitive to platinum agents (CH1: JM216 4 h IC50 of 5.8 microM) and the other relatively resistant (SKOV3: JM216 4 h IC50 of 60.7 microM). Metabolic profiles were also evaluated at different growth status and in cells pretreated with buthionine sulphoximine (BSO), an agent known to decrease intracellular glutathione levels. Results showed that JM216 enters the cells and that the nature and percentage of biotransformation products was dependent upon glutathione levels. Furthermore, results support the view that the previously reported peak A biotransformation product contains a glutathione adduct. In exponentially growing SKOV3 cells which contain higher glutathione levels than CH1, (82.5 vs 37.8 nmol mg-1 protein), peak A represented 89% of total platinum 4 h after JM216 exposure compared with only 24% in CH1. Moreover, 60-70% depletion of glutathione achieved by 24 h pretreatment of cells with BSO resulted in a significant decrease in peak A in both cell lines and increased the cytotoxicity of JM216 in both CH1 and SKOV3 by approximately 2-fold. Following a 4 h exposure of exponentially growing SKOV3 cells to JM216, only peak A (89%) and JM216 (11%) could be detected whereas in CH1 cells, peak A (24%), JM216 (73%) and JM118 [cis-ammine dichloro (cyclohexylamine) platinum II] (3%) were detected. However, in CH1 cells at confluence, where glutathione is lower (8 nmol mg-1 protein) four metabolites (plus JM216 itself) were detected following exposure to 50 microM JM216; peak A, JM118, JM383 (bis-acetato ammine (cyclohexylamine) dihydroxy platinum IV) and an unidentified metabolite (D), also observed in patient's plasma ultrafiltrate. In confluent SKOV3 cells exposed to 50 microM JM216, peak A, JM216 and JM118 were detected. A further unidentified metabolite observed in patients receiving JM216 (metabolite F) was not formed inside these tumour cells. Overall, these data suggest that glutathione conjugation represents a major deactivation pathway for JM216.

Synthesis and X-ray crystal structure of trans,cis-[Pt(OAc)2I2(en)]: a novel type of cisplatin analog that can be photolyzed by visible light to DNA-binding and cytotoxic species in vitro

An original approach intended to facilitate the intratumoral activation of Pt(IV) diamines by illumination with visible light to form photolysis products that irreversibly bind to DNA and are cytotoxic to human cancer cells is reported. The novel Pt(IV) complex trans,cis-[Pt(OAc)2I2-(en)] was prepared by the acetylation of trans,cis-[Pt(OH)2I2(en)] with acetic anhydride in CH2-Cl2; trans,cis-[Pt(OH)2I2(en)] was synthesized by oxidation of [PtI2(en)] with 30% aqueous H2O2. trans,cis-[Pt(OAc)2I2(en)] crystallized from methanol as deep-red needles with a = 9.029(4) A, b = 11.443(2) A, c = 12.822(2) A, beta = 95.48(3) degrees, monoclinic space group Cc, and Z = 4. The conformation of the acetato groups around the O-Pt-O axis deviated significantly from the conformation of the acetato groups in the X-ray crystal structure reported for the cis-dichloro analog, which may explain the very different aqueous solubilities of the two compounds. trans,-cis-[Pt(OAc)2I2(en)] and trans,cis-[Pt(OH)2I2(en)] displayed broad ligand-to-metal charge-transfer bands centered at lambda = 389 and 384 nm, respectively (epsilon = 1372 and 1425 M-1 cm-1, respectively), with tailing out to ca. 550 nm. When trans,cis-[Pt(OAc)2I2(en)] was incubated with calf thymus DNA in the absence of light, no covalent binding of Pt to DNA was measurable after 6 h; however, irradiation with light of wavelengths > 375 nm resulted in 63 +/- 13% of the platinum being covalently bound to DNA after 6 h, suggesting that a photoreduction to Pt(II) species took place. Although trans,cis-[Pt(OH)2I2(en)] was also labile to visible light, only 10 +/- 2% DNA platination was observed after 6 h of illumination; however, covalent binding of Pt to DNA took place quantitatively when a reducing agent such as glutathione was added to the photolyzed incubations. These results provide evidence that the photolysis of the trans-dihydroxo analog resulted predominately in the substitution of the iodide ligands for water rather than a reduction of Pt(IV) to Pt(II). When protected from light, trans,cis-[Pt(OAc)2I2-(en)] and trans,cis-[Pt(OH)2I2(en)], both at a concentration of 10 microM, had half-lives of 6.6 +/- 0.5 and 46.8 +/- 8.8 h, respectively, at 37 degrees C in Eagle's minimum essential medium (EMEM) containing 5% fetal calf serum. When irradiated with light lambda(irr) > 375 nm, the half-lives were decreased by 24- and 53-fold for the diacetato- and dihydroxoplatinum(IV) complexes, respectively. Compared to the "dark" control, the in vitro treatment of TCCSUP human bladder cancer cells with trans,cis-[Pt(OAc)2I2(en)] resulted in 35% greater growth inhibitory activity when during the first 1.5 h of drug exposure the cells were irradiated with light lambda irr > 375 nm. The photolysis of trans,cis-[Pt(OH)2I2(en)] with visible light resulted in a 22% enhancement of antiproliferative activity.