Polymeric iron chelators for enhancing 5-aminolevulinic acid-induced photodynamic therapy.

5-Aminolevulinic acid (5-ALA) is an amino acid that can be metabolized into a photosensitizer, protoporphyrin IX (PpIX) selectively in a tumor cell, permitting minimally invasive photodynamic diagnosis/therapy. However, some malignant tumor cells have excess intracellular labile iron and facilitate the conversion of PpIX into heme, which compromises the therapeutic potency of 5-ALA. Here, we examined the potential of chelation of such unfavorable intratumoral labile iron in photodynamic therapy (PDT) with 5-ALA hydrochloride, using polymeric iron chelators that we recently developed. The polymeric iron chelator efficiently inactivated the intracellular labile iron in cultured cancer cells and importantly enhanced the accumulation of PpIX, thereby improving the cytotoxicity upon photoirradiation. Even in in vivo study with subcutaneous tumor models, the polymeric iron chelator augmented the intratumoral accumulation of PpIX and the PDT effect. This study suggests that our polymeric iron chelator could be a tool for boosting the effect of 5-ALA-induced PDT by modulating tumor microenvironment.

Current status of photodynamic technology for urothelial cancer.

5-Aminolevulinic acid is a new-generation photosensitizer with high tumor specificity. It has been used successfully in the diagnosis, treatment, and screening of urological cancers including bladder cancer; specifically, it has been used in photodynamic diagnosis to detect tumors by illuminating the lesion with a specific wavelength of light to produce fluorescence in the lesion after administration of 5-aminolevulinic acid, in photodynamic therapy, which induces tumor cell death via production of cytotoxic reactive oxygen species, and in photodynamic screening, in which porphyrin excretion in the blood and urine is used as a tumor biomarker after administration of 5-aminolevulinic acid. In addition to these applications in urological cancers, 5-aminolevulinic acid-based photodynamic technology is expected to be used as a novel strategy for a large number of cancer types because it is based on a property of cancer cells known as the Warburg effect, which is a basic biological property that is common across all cancers.

4'-Nitrobiphenyl thioglucoside as the Smallest, fluorescent photosensitizer with cancer targeting ligand.

We have previously developed a glucose-linked biphenyl photosensitizer that can pass through glucose transporters, aiming for cancer-selective photodynamic therapy (PDT). Its small size (MW: 435) will allow oral administration and a fast clearance avoiding photosensitivity. However, its fluorescence efficiency was quite low, causing difficulty in monitoring cellular uptake. We thus synthesized a series of monosaccharide-linked biphenyl derivatives with a sulfur atom replacing an oxygen atom, in search of a photosensitizer with a brighter fluorescence. Among them, 4'-nitrobiphenyl thioglucoside showed a fluorescence emission extending to near infra-red region with a strength three times greater than that of the previous compound. This compound was found to have a higher 1O2-producing efficiency (ΦΔ: 0.75) than the previous compound (ΦΔ: 0.65). The thioglucoside indicated a significant photodamaging effect (IC50: 250 µM) against cancer cells. Although the galactose and mannose analogs exerted similar photodamaging effects, they were moderately toxic in the dark at a concentration of 300 µM. The thioglucoside and thiomannoside were at least partially uptaken through glucose transporters as demonstrated by inhibition with cytochalasin B, whereas no inhibition was observed for the galactoside. The behavior of d-glucose toward the cellular uptakes of these photosensitizers was bipolar: inhibitory at a low concentration and recovery or acceleratory at a higher concentration. These results indicate that 4'-nitrobiphenyl thioglucoside is the smallest (MW: 393) cancer-targeting photosensitizer with a trackable fluorescence property.

Key transporters leading to specific protoporphyrin IX accumulation in cancer cell following administration of aminolevulinic acid in photodynamic therapy/diagnosis.

The administration of aminolevulinic acid allow the formation and accumulation of protoporphyrin IX specifically in cancer cells, which then lead to photocytotoxicity following light irradiation. This compound, when accumulated at high levels, could also be used in cancer diagnosis as it would emit red fluorescence when being light irradiated. The concentration of protoporphyrin IX is pivotal in ensuring the effectiveness of the therapy. Studies have been carried out and showed the importance of various transporters in regulating the amount of these substrates by controlling the transport of various related metabolites in and out of the cell. There are many transporters involved and their expression levels are dependent on various factors, such as oxygen availability and iron ions. It is also important to note that these transporters may also have different expression levels depending on their organ. Understanding the mechanisms and the roles of these transporters are essential to ensure maximum accumulation of protoporphyrin IX, leading to higher efficiency in photodynamic therapy/diagnosis. In this review, we would like to discuss the roles of various transporters in protoporphyrin IX accumulation and how their involvement directly affect cancerous microenvironment.

5-Aminolevulinic acid regulates the immune response in LPS-stimulated RAW 264.7 macrophages.

BACKGROUND: Macrophages are crucial players in a variety of inflammatory responses to environmental cues. However, it has been widely reported that macrophages cause chronic inflammation and are involved in a variety of diseases, such as obesity, diabetes, metabolic syndrome, and cancer. In this study, we report the suppressive effect of 5-aminolevulinic acid (ALA), via the HO-1-related system, on the immune response of the LPS-stimulated mouse macrophage cell line RAW264.7. RESULTS: RAW264.7 cells were treated with LPS with or without ALA, and proinflammatory mediator expression levels and phagocytic ability were assessed. ALA treatment resulted in the attenuation of iNOS and NO expression and the downregulation of proinflammatory cytokines (TNF-α, cyclooxygenase2, IL-1ß, IL-6). In addition, ALA treatment did not affect the phagocytic ability of macrophages. To our knowledge, this study is the first to investigate the effect of ALA on macrophage function. Our findings suggest that ALA may have high potential as a novel anti-inflammatory agent. CONCLUSIONS: In the present study, we showed that exogenous addition of ALA induces HO-1 and leads to the downregulation of NO and some proinflammatory cytokines. These findings support ALA as a promising anti-inflammatory agent.

Water-Soluble Glucosyl Pyrene Photosensitizers: An Intramolecularly Synthesized 2- C-Glucoside and an O-Glucoside.

Prevalent photosensitizing agents for photodynamic therapy (PDT) suffer from their relatively large molecular weights causing photodermatosis. In this regard, low molecular weight pyrene could be an efficient photosensitizer except for its extreme hydrophobicity. To tackle the insolubility of pyrene, we synthesized 1-carboxypyren-2-yl C-glucoside 4 by a tethered C-glucosylation and 1-pyrenylmethyl O-glucoside 5 by a simple O-glucosylation. Compounds 4 and 5 showed modest water solubilities of 72 and 47 µg/mL, respectively. Whereas compound 4 partially underwent a cyclization reaction at pH 3 to give the corresponding δ-valerolactone 15b in 31% yield after 24 h, it is stable at pH 5-9 for at least a week. The 1O2-producing photosensitizabilities of 4 and 5 were sufficient to apply to PDT. Although compound 5 was uptaken by HeLa cells and showed a good PDT activity, compound 4 showed neither a sufficient cell uptake nor PDT effect. The binding modes of compounds 4 and 5 to concanavalin A were specific and unspecific, respectively. These results demonstrate that compounds 4 and 5 are within a pharmacologically acceptable range as oral drugs and could be a fluorescence imaging probe for α-glucose/mannose receptors and a photosensitizing agent for PDT, respectively.

Coating lanthanide nanoparticles with carbohydrate ligands elicits affinity for HeLa and RAW264.7 cells, enhancing their photodamaging effect.

Lanthanide nanoparticles (LNPs) conjugated with monosaccharides were synthesized as a photon energy-upconverting nanodevice with affinity to cancer cells. The conjugates were designed to selectively damage the cancer cells containing protoporphyrin IX, a photosensitizer endogenously synthesized from priorly administrated 5-aminolevlunic acid (ALA), by a highly tissue-penetrative near-infrared (NIR) irradiation. First of all, the affinities of monosaccharides toward cells (HeLa, RAW264.7, and MKN45) were assessed by a novel cell aggregation assay with trivalent monosaccharide-citric acid conjugates. As a result, HeLa exhibited high affinity for glucose, while RAW264.7 for glucose, galactose, mannose, and fucose. A similar cell-monosaccharide affinity was microscopically observed when the cells were mixed with monosaccharide-LNP conjugates and rinsed, in which the high affinity LNP probes luminesced on the cells. The high affinity monosaccharide-LNPs showed greater photodamaging effects than the unmodified LNP toward the corresponding cells, when the cells were pretreated with ALA and irradiated by NIR. This study demonstrates that carbohydrates can be used as selective ligands for cancer cells in a photodynamic therapy with LNP.

Silencing of ATPase Inhibitory Factor 1 Inhibits Cell Growth via Cell Cycle Arrest in Bladder Cancer.

OBJECTIVE: The role of the ATPase inhibitory factor 1 (IF1) is inhibit the hydrolase activity of F1Fo-ATPase when oxidative phosphorylation is impaired. It has been demonstrated that IF1 is overexpressed in various carcinomas and mediates tumor cell activities, but the detailed mechanisms of IF1-mediated tumor progression and the link between IF1 and cell cycle progression remain unclear. Herein, we aimed to investigate the potential role of IF1 in cell cycle progression of human bladder cancer (BCa). METHODS: The expression of IF1 was analyzed by immunohistochemistry in tumor tissues. Western blot was used to detect protein expression in the cells. Cell proliferation was determined by MTT and colony formation assays. The cell cycle was analyzed using flow cytometry. RESULTS: We firstly showed IF1 was overexpressed in BCa. Silencing of IF1 by small interfering RNA led to a significant decrease in cell proliferation and migration in T24 and UM-UC-3 cells. Importantly, IF1 knockdown caused cell cycle arrest at G0/G1 stage and decreased the protein level of cyclin E/cyclin-dependent kinases (cdk) 2 and/or cyclin D/cdk4/cdk6. CONCLUSION: These results suggest the inhibitory effect of IF1 knockdown on BCa cell proliferation is via the suppression of cyclins and cdks related to G1/S transition and then induction of G0/G1 arrest, and firstly indicate IF1 mediates the tumor cell cycle. We concluded that IF1 may be a novel therapeutic target for BCa.

5-Aminolevulinic acid enhances cell death under thermal stress in certain cancer cell lines.

5-aminolevulinic acid (5-ALA) is contained in all organisms and a starting substrate for heme biosynthesis. Since administration of 5-ALA specifically leads cancer cells to accumulate protoporphyrin IX (PpIX), a potent photosensitizer, we tested if 5-ALA also serves as a thermosensitizer. 5-ALA enhanced heat-induced cell death of cancer cell lines such as HepG2, Caco-2, and Kato III, but not other cancer cell lines including U2-OS and normal cell lines including WI-38. Those 5-ALA-sensitive cancer cells, but neither U2-OS nor WI-38, accumulated intracellular PpIX and exhibited an increased reactive oxygen species (ROS) generation under thermal stress with 5-ALA treatment. In addition, blocking the PpIX-exporting transporter ABCG2 in U2-OS and WI-38 cells enhanced their cell death under thermal stress with 5-ALA. Finally, a ROS scavenger compromised the cell death enhancement by 5-ALA. These suggest that 5-ALA can sensitize certain cancer cells, but not normal cells, to thermal stress via accumulation of PpIX and increase of ROS generation.

The Effect of Coatings on the Affinity of Lanthanide Nanoparticles to MKN45 and HeLa Cancer Cells and Improvement in Photodynamic Therapy Efficiency.

An improvement in photodynamic therapy (PDT) efficiency against a human gastric cancer cell line (MKN45) with 5-aminolevulinic acid (ALA) and lanthanide nanoparticles (LNPs) is described. An endogenous photosensitizer, protoporphyrin IX, biosynthesized from ALA and selectively accumulated in cancer cells, is sensitizable by the visible lights emitted from up-conversion LNPs, which can be excited by a near-infrared light. Ten kinds of surface modifications were performed on LNPs, NaYF4(Sc/Yb/Er) and NaYF4(Yb/Tm), in an aim to distribute these irradiation light sources near cancer cells. Among these LNPs, only the amino-functionalized LNPs showed affinity to MKN45 and HeLa cancer cells. A PDT assay with MKN45 demonstrated that amino-modified NaYF4(Sc/Yb/Er) gave rise to a dramatically enhanced PDT effect, reaching almost perfect lethality, whereas NaYF4(Yb/Tm)-based systems caused little improvement in PDT efficiency. The improvement of PDT effect with the amino-modified NaYF4(Sc/Yb/Er) is promising for a practical PDT against deep cancer cells that are reachable only by near-infrared lights.

Cytoreductive surgery under aminolevulinic acid-mediated photodynamic diagnosis plus hyperthermic intraperitoneal chemotherapy in patients with peritoneal carcinomatosis from ovarian cancer and primary peritoneal carcinoma: results of a phase I trial.

BACKGROUND: We conducted a phase I clinical trial to evaluate the sensitivity, specificity, and safety of cytoreductive surgery (CRS) under aminolevulinic acid-mediated photodynamic diagnosis (ALA-PDD) plus hyperthermic intraperitoneal chemotherapy (HIPEC) on 20 patients with peritoneal carcinomatosis (PC) from ovarian cancer and primary peritoneal carcinoma (PPC). PATIENTS AND METHODS: Patients took 5-aminolevulinic acid (5-ALA) at a dose of 20 mg/kg orally with 50 mL of water 2 h before surgery. During surgery, the abdominal cavity was observed under blue light (wavelength of 440 nm) before and after CRS plus HIPEC. Specimens were excised and submitted for pathological examination to evaluate the specificity of ALA-PDD. Postoperative course was closely monitored and detailed information was recorded. RESULTS: CRS under ALA-PDD plus HIPEC was performed 21 times in 20 patients with PC (16 ovarian cancer, 4 PPC) between June 2011 and October 2013. With the exception of 1 (5 %) patient, strong red fluorescence was detected in 19 patients with ovarian cancer, with a sensitivity of 95 %. All specimens from red fluorescent lesions were invaded by cancer cells, with a specificity of 100 %. No severe adverse events occurred during the perioperative period, with the exception of some abnormal laboratory results and mild complications. All patients were alive until the last follow-up. CONCLUSION: ALA-PDD provided a high sensitivity and specificity in detecting peritoneal metastasis in patients with PC from ovarian serous carcinoma and PPC. CRS under ALA-PDD plus HIPEC was a feasible and safe treatment option for patients with PC from ovarian cancer and PPC.

Photodynamic detection and management of intraperitoneal spreading of primary peritoneal papillary serous carcinoma in a man: report of a case.

As a peritoneal surface malignancy, primary peritoneal papillary serous carcinoma (PPPSC) almost always occurs in women. Our search of the literature found only two previous case reports of men with PPPSC, both with very short survival. We report the case of a 63-year-old man with PPPSC, treated effectively with cytoreductive surgery and docetaxel-based hyperthermic intraperitoneal chemotherapy following six cycles of docetaxel-based laparoscopic neoadjuvant intraperitoneal and cisplatin-based systemic chemotherapy. Furthermore, we detected intraoperative intraperitoneal spreading of the tumor after the oral administration of 5-amino levulinic acid (5-ALA). The patient remains in good health without ascites 18 months after his diagnosis. Thus, primary peritoneal papillary serous carcinoma should be managed by intraperitoneal chemotherapy combined with peritonectomy procedures. Moreover, the intraoperative detection of the intraperitoneal spreading of the tumor after administering oral 5-ALA shows that this is an exciting and promising diagnostic technique, which needs to be confirmed by further studies.

Hemin enhances the 5-aminolevulinic acid-photodynamic therapy effect through the changes of cellular iron homeostasis.

BACKGROUND: Photodynamic therapy (PDT) has been utilized as a promising alternative cancer treatment due to its minimum invasiveness over the years. Exogenous 5-aminolevulinic acid (ALA) triggers protoporphyrin IX (PpIX) accumulation, which happens in cancer cells. However, certain types of cancer exhibit reduced effectiveness in the PpIX accumulation mechanism. This study aimed to determine the effect of ALA-PDT combination with hemin on gastric carcinoma TMK-1 cells. METHODS: This study utilized TMK-1 gastric cancer cell line to evaluate PpIX, ROS, and Fe2+ accumulation following the administration of ALA, hemin, and a combination of ALA and hemin PDT. We also evaluate the mRNA expressions related to iron homeostasis and treatment impacts on cell viability. RESULTS: The co-addition of ALA and hemin PDT for 4 h of treatment resulted in a significant decrease in cell viability by up to 18 %. While ALA-PDT enhanced PpIX metabolism, the addition of hemin influenced both the production of reactive oxygen species (ROS) and cellular iron homeostasis by inducing Fe2+ accumulation and affecting mRNA levels of IRP, Tfr1, Ferritin, NFS1, and SDHB. CONCLUSION: These findings suggest that the addition of ALA and hemin enhances phototoxicity in TMK-1 cells. The combination of ALA and hemin with PDT induces cell death, evidenced by increased cytotoxicity properties such as PpIX and ROS, along with significant changes in TMK-1 gastric cancer iron homeostasis. Therefore, the combination of ALA and hemin could be one of the alternatives in photodynamic therapy for cancer in the future.

Sugar-attached upconversion lanthanide nanoparticles: a novel tool for high-throughput lectin assay.

To create a novel high-throughput lectin assay (HTPLA) method based on the emission of a luminophore by highly penetrable near-infrared excitation, sugar-attached upconversion lanthanide nanoparticles (LNPs) were synthesized as a tool to highlight the aggregates caused by the sugar-mediated specific bridging between LNP and lectin. The emissions from a mannose-coated LNP in the aggregates with a mannose-binding lectin were much stronger than those from the non-aggregated samples, being sensitive enough for HTPLA. A galactose-coated LNP was also applicable to a macrophage aggregation assay for the sugar specificity of its surface lectin.

Cell senescence-associated porphyrin metabolism affects the efficacy of aminolevulinic acid-photodynamic diagnosis in bladder cancer.

Aminolevulinic acid-photodynamic diagnosis (ALA-PDD) is a promising alternative method to detect cancer cells because of its high specificity and low rate of side effects. Exogenous ALA is administered and accumulates as protoporphyrin IX (PpIX) in cancer cells, which then emit red fluorescence following light irradiation to enable surgeons to accurately identify and remove cancerous tissue. Recent reports suggested that PpIX failed to accumulate in some patients who underwent ALA-PDD. We hypothesized that cell senescence, which is a relatively inactive state, affects porphyrin accumulation in bladder cancer cells. In this study, we evaluated the relationship between cell senescence and porphyrin accumulation in affecting the efficacy of ALA-PDD. First, we utilized three bladder cancer cell lines to evaluate senescence-related indicators and establish a cell senescence model. Then, we identified the differences in porphyrin production and the proteins involved in porphyrin accumulation between old and young cells. We found that compared with young cells, old cells possessed higher concentration of PpIX and had lower ABCG2 expression. The increase in PpIX levels following ABCG2 inhibition is three times higher in old cells than in young cells, suggesting that cell senescence was closely related with porphyrin accumulation in cancer. In conclusion, we found that the efficacy of ALA-PDD and porphyrin accumulation was relatively high in senescent cancer cells and that inhibition of ABCG2 could improve the efficacy of ALA-PDD in young bladder cancer cells.

Suppression of angiotensin converting enzyme 2, a host receptor for SARS-CoV-2 infection, using 5-aminolevulinic acid in vitro.

Angiotensin converting enzyme 2 (ACE2), an entry receptor found on the surface of host cells, is believed to be detrimental to the infectious capability of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). Scientists have been working on finding a cure since its outbreak with limited success. In this study, we evaluated the potential of 5-aminolevulinic acid hydrochloride (ALA) in suppressing ACE2 expression of host cells. ACE2 expression and the production of intracellular porphyrins following ALA administration were carried out. We observed the reduction of ACE2 expression and intracellular porphyrins following ALA administration. ALA suppressed the ACE2 expression in host cells which might prevent binding of SARS-CoV-2 to host cells. Co-administration of ALA and sodium ferrous citrate (SFC) resulted in a further decrease in ACE2 expression and increase in intracellular heme level. This suggests that the suppression of ACE2 expression by ALA might occur through heme production. We found that the inhibition of heme oxygenase-1 (HO-1), which is involved in heme degradation, also resulted in decrease in ACE2 expression, suggesting a potential role of HO-1 in suppressing ACE2 as well. In conclusion, we speculate that ALA, together with SFC administration, might serve as a potential therapeutic approach in reducing SARS-CoV-2 infectivity through suppression of ACE2 expression.

Metabolic shift towards oxidative phosphorylation reduces cell-density-induced cancer-stem-cell-like characteristics in prostate cancer in vitro.

Numerous cancer patients undergoing conventional cancer therapies such as radiotherapy, chemotherapy and surgical tumour removal face relapses several years or even decades later. This may be due to the presence of cancer stem cells (CSCs) that survived said therapies. In this study, we aimed to uncover the relationship between cell density and CSCs, and the role of the Warburg effect in regulating CSC-like characteristics. A prostate cancer cell line, PC3, was used in this study. To investigate the Warburg effect effect and CSC-like characteristics in prostate cancer, we measured the expression levels of glycolysis and OXPHOS-related genes, and performed spheroid forming, cell viability and various glycolysis and OXPHOS-assays. We observed that increased cell density caused a metabolic shift from glycolysis to OXPHOS and higher CSC-like characteristics. However, the use of dichloroacetate (DCA), an inhibitor of the Warburg effect, significantly inhibited the cell-density-induced metabolic shift and CSC-like characteristics. Changes in cell density strongly influenced the preferred metabolic pathway of prostate cancer cells, regulating their CSC-like characteristics. It is possible that DCA, an inhibitor of the Warburg effect, could be a novel drug used to treat CSCs by distinguishing Warburg effect, preventing future cancer relapses.

Mangostin enhances efficacy of aminolevulinic acid-photodynamic therapy against cancer through inhibition of ABCG2 activity.

BACKGROUND: Aminolevulinic acid-photodynamic therapy (ALA-PDT) is gaining attention as a potential method for treating select cancers due to its high specificity and low side effect feature. ALA enters cancer cells and accumulate as protoporphyrin IX (PpIX), which will then trigger phototoxicity following light irradiation. However, it is reported that some cancer cells have reduced efficacy of ALA-PDT due to high expression of ABCG2, a transporter involved in the PpIX efflux. In this study, we evaluated the effect of mangostin, a natural compound containing anti-tumor property, on the efficacy of ALA-PDT against cancer and the mechanism involved. METHODS: We utilized TMK1 gastric cancer cell line, which has high ABCG2 expression, to evaluate the PpIX accumulation and phototoxicity exerted by ALA and mangostin co-addition. RESULTS: We found that co-addition of ALA and mangostin significantly increase the phototoxicity and PpIX accumulation in TMK1 cells. We also investigated the effect of mangostin on porphyrin-heme pathway enzymes and ABCG2 and found that the addition of mangostin reduce the activity of ABCG2, reducing PpIX efflux. CONCLUSION: These findings suggest that mangostin enhances the efficacy of ALA-PDT in cancer through inhibition of ABCG2 activity.

Novel protein extraction approach using micro-sized chamber for evaluation of proteins eluted from formalin-fixed paraffin-embedded tissue sections.

We describe a novel antigen-retrieval method using a micro-sized chamber for mass spectrometry (MS) analysis to identify proteins that are preferentially eluted from formalin-fixed paraffin-embedded (FFPE) samples. This approach revealed that heat-induced antigen retrieval (HIAR) from an FFPE sample fixed on a glass slide not only improves protein identification, but also facilitates preferential elution of protein subsets corresponding to the properties of antigen-retrieval buffers. Our approach may contribute to an understanding of the mechanism of HIAR.

Syntheses, characterization, and antitumor activities of platinum(II) and palladium(II) complexes with sugar-conjugated triazole ligands.

Four platinum(II) and palladium(II) complexes with sugar-conjugated bipyridine-type triazole ligands, [Pt(II)Cl(2)(AcGlc-pyta)] (3), [Pd(II)Cl(2)(AcGlc-pyta)] (4), [Pt(II)Cl(2)(Glc-pyta)] (5), and [Pd(II)Cl(2)(Glc-pyta)] (6), were prepared and characterized by mass spectrometry, elemental analysis, (1)H- and (13)C-NMR, IR as well as UV/VIS spectroscopy, where AcGlc-pyta and Glc-pyta denote 2-[4-(pyridin-2-yl)-1H-1,2,3-triazol-1-yl]ethyl 2,3,4,6-tetra-O-acetyl-ß-D-glucopyranoside (1) and 2-[4-(pyridin-2-yl)-1H-1,2,3-triazol-1-yl]ethyl ß-D-glucopyranoside (2), respectively. The solid-state structure of complex 6 was determined by single-crystal X-ray-diffraction analysis. These complexes exhibited in vitro cytotoxicity against human cervix tumor cells (HeLa) though weaker than that of cisplatin.