Satraplatin in Hormone-Refractory Prostate Cancer and Other Tumour Types

Activation of Platinum(IV) Prodrugs by Cytochrome c and Characterization of the Protein Binding Sites

Platinum(IV) complexes generally require reduction to reactive Pt(II) species to exert their chemotherapeutic activity. The process of reductive activation of (15)N-labeled (OC-6-43)-bis(acetato)diamminedichloridoplatinum(IV), in the presence of nicotinamide adenine dinucleotide (NADH) and horse heart cytochrome c (cyt c), was monitored by (1)H,(15)N-HSQC NMR spectroscopy and protein digestion experiments. It has been shown that cyt c plays a catalytic role in the transfer of two reducing equivalents from NADH to Pt(IV) species. Noncovalent interactions between reduced monoaqua cisplatin (cis-[PtCl((15)NH3)2(H2O)](+)) and the protein, in the proximity of the heme cofactor, and also covalent binding of platinum to the protein region around Met65 and Met80 take place.

The plasma and cerebrospinal fluid pharmacokinetics of the platinum analog satraplatin after intravenous administration in non-human primates

BACKGROUND

Satraplatin is an orally bioavailable platinum analog with preclinical activity in cisplatin resistant models and clinical activity in adults with refractory cancers. The cerebrospinal fluid (CSF) penetration of cisplatin and carboplatin in non-human primates (NHP) is limited (3.7 and 2.6%, respectively). We evaluated the plasma and CSF pharmacokinetics (PK) of satraplatin after an intravenous (IV) dose in NHP.

METHODS

Satraplatin (120 mg/m(2)) was administered as 1 h IV infusion in DMSO (5%) and normal saline to 5 NHP. Serial blood and CSF samples were obtained over 48 h. Plasma ultrafiltrate (UF) was immediately prepared by centrifugation. Platinum was quantified in plasma UF and CSF using a validated atomic absorption spectroscopy assay with lower limit of quantification (LLQ) of 0.025 μM in UF and 0.006 μM after concentration in CSF. Pharmacokinetic parameters were estimated using non-compartmental analyses. CSF penetration was calculated from the CSF AUC(0-48h) : plasma UF AUC(0-48h).

RESULTS

Satraplatin was well tolerated. Median (range) PK parameters in plasma UF were: maximum concentration (C (max)) 8.3 μM (5.7-10.6), area under the curve (AUC(0-48h)) 29.2 μM h (22.6-33.2), clearance 0.36 l/h/kg (0.31-0.37), and t (1/2) 18.8 h (13.4-25). Satraplatin was detected in the CSF of all NHP. Median (range) PK parameters in CSF were: C (max) 0.07 μM (0.02-0.12), AUC(0-48h) 1.2 μM h (0.49-2.43). The median (range) CSF penetration of satraplatin was 4.3% (2.2-7.4).

CONCLUSIONS

Satraplatin penetration into CSF is similar to that of carboplatin and cisplatin, despite its greater lipophilicity. The development of a phase I trial of satraplatin for refractory childhood solid tumors including brain tumors is in progress.

Clinical and pharmacokinetic evaluation of satraplatin

INTRODUCTION

The toxicities of cisplatin, that is, nephrotoxicity, neurotoxicity and emesis, provided the impetus for the development of more tolerable platinum analogs. Satraplatin is an investigational third-generation orally available lipophilic platinum, which has demonstrated safety and antitumor activity in multiple settings.

AREAS COVERED

The clinical activity of satraplatin in metastatic castrate-resistant prostate cancer (mCRPC), breast, lung and other advanced solid tumors is discussed with a focus on its pharmacokinetic properties. The article was formulated using publications found through PubMed search in addition to presentations given at major conferences.

EXPERT OPINION

Satraplatin was associated with dose-limiting myelosuppression, but no significant ototoxicity, neurotoxicity or nephrotoxicity. Despite the activity of satraplatin in mCRPC, survival was not extended in an unselected population included in a Phase III trial. While further development of satraplatin in large Phase III trials is not planned at this time, efforts are ongoing to develop tailored therapy in mCRPC based on excision repair cross-complementing group 1 expression or BRCAness. Moreover, based on potentially better central nervous system penetration due to lipophilicity, evaluation in patients with brain tumors is ongoing. Given the favorable toxicity profile and convenient oral administration, satraplatin may warrant development in settings that preclude cisplatin, for example, underlying renal dysfunction, elderly age and poor performance status.

Drug transporters of platinum-based anticancer agents and their clinical significance

Platinum-based drugs are among the most active anticancer agents and are successfully used in a wide variety of human malignancies. However, acquired and/or intrinsic resistance still represent a major limitation. Lately, in particular mechanisms leading to impaired uptake and/or decreased cellular accumulation of platinum compounds have attracted attention. In this review, we focus on the role of active platinum uptake and efflux systems as determinants of platinum sensitivity and -resistance and their contribution to platinum pharmacokinetics (PK) and pharmacodynamics (PD). First, the three mostly used platinum-based anticancer agents as well as the most promising novel platinum compounds in development are put into clinical perspective. Next, we describe the presently known potential platinum transporters--with special emphasis on organic cation transporters (OCTs)--and discuss their role on clinical outcome (i.e. efficacy and adverse events) of platinum-based chemotherapy. In addition, transporter-mediated tumour resistance, the impact of potential platinum transporter-mediated drug-drug interactions, and the role of drug transporters in the renal elimination of platinum compounds are discussed.

Mechanistic insight into the inhibition of matrix metalloproteinases by platinum substrates

Platinum compounds are among the most used DNA-damaging anticancer drugs, however they can also be tailored to target biological substrates different from DNA, for instance enzymes involved in cancer progression. We recently reported that some platinum complexes with three labile ligands inhibit matrix metalloproteinase activity in a selective way. We have now extended the investigation to a series of platinum complexes having three chlorido or one chlorido and a dimethylmalonato leaving ligands. All compounds are strong inhibitors of MMP-3 by a noncompetitive mechanism, while platinum drugs in clinical use are not. Structural investigations reveal that the platinum substrate only loses two labile ligands, which are replaced by an imidazole nitrogen of His224 and a hydroxyl group, while it retains one chlorido ligand. A chlorido and a hydroxyl group are also present in the zinc complex inhibitor of carboxypeptidase A, whose active site has strong analogies with that of MMP-3.

RHAMM (CD168) is overexpressed at the protein level and may constitute an immunogenic antigen in advanced prostate cancer disease

Localized prostate cancer (CaP) can be cured using several strategies. However, the need to identify active substances in advanced tumor stages is tremendous, as the outcome in such cases is still disappointing. One approach is to deliver human tumor antigen-targeted therapy, which is recognized by T cells or antibodies. We used data mining of the Cancer Immunome Database (CID), which comprises potential immunologic targets identified by serological screening of expression libraries. Candidate antigens were screened by DNA microarrays. Genes were then validated at the protein level by tissue microarrays, representing various stages of CaP disease. Of 43 targets identified by CID, 10 showed an overexpression on the complementary DNA array in CaP metastases. The RHAMM (CD168) gene, earlier identified by our group as an immunogenic antigen in acute and chronic leukemia, also showed highly significant overexpression in CaP metastases compared with localized disease and benign prostatic hyperplasia. At the protein level, RHAMM was highest in metastatic tissue samples and significantly higher in neoplastic localized disease compared with benign tissue. High RHAMM expression was associated with clinical parameters known to be linked to better clinical outcome. Patients with high RHAMM expression in the primaries had a significantly lower risk of biochemical failure. The number of viable cells in cell cultures was reduced in blocking experiments using hormone-sensitive and hormone-insensitive metastatic CaP cell lines. Acknowledging the proven immunogenic effects of RHAMM in leukemia, this antigen is intriguing as a therapeutic target in far-advanced CaP.

Polymeric drug delivery of platinum-based anticancer agents

Platinum-based anticancer agents such as cisplatin and carboplatin are in widespread clinical use but associated with many side effects. Improving the delivery of cytotoxic platinum compounds may lead to reduced side effects and achieve greater efficacy at lower doses. Polymer-based therapeutics have been investigated as potential drug delivery vehicles for platinum-based drugs. Against a background of the chemistry and pharmacology of cytotoxic platinum compounds, this review discusses the formation and properties of platinum-polymer complexes, dendrimers, micelles, and microparticulates.

Comparative protein binding, stability and degradation of satraplatin, JM118 and cisplatin in human plasma in vitro

1. Satraplatin is an investigational orally administered platinum-based antitumour drug. The present study compared the plasma protein binding, stability and degradation of satraplatin with that of its active metabolite JM118 and cisplatin. 2. The platinum complexes were incubated in human plasma for up to 2 h at 37 degrees C and quantified in plasma fractions by inductively coupled plasma-mass spectrometry on- or off-line to high-performance liquid chromatography. 3. All three platinum drugs became irreversibly bound to plasma proteins and showed negligible reversible protein binding. They were also unstable in plasma and generated one or more platinum-containing degradation products during their incubation. However, the three platinum complexes differed in the kinetics of their instability and protein binding, as well as in the number of degradation products formed during their incubation. 4. In conclusion, the plasma protein binding, instability and degradation of satraplatin and its active metabolite JM118 are qualitatively similar to that of cisplatin and other clinically approved platinum-based drugs. Quantitative differences in their irreversible protein binding and degradation were related to their respective physiochemical properties and bioactivation mechanisms.

The resurgence of platinum-based cancer chemotherapy

The accidental discovery of the anticancer properties of cisplatin and its clinical introduction in the 1970s represent a major landmark in the history of successful anticancer drugs. Although carboplatin--a second-generation analogue that is safer but shows a similar spectrum of activity to cisplatin--was introduced in the 1980s, the pace of further improvements slowed for many years. However, in the past several years interest in platinum drugs has increased. Key developments include the elucidation of mechanisms of tumour resistance to these drugs, the introduction of new platinum-based agents (oxaliplatin, satraplatin and picoplatin), and clinical combination studies using platinum drugs with resistance modulators or new molecularly targeted drugs.