Manganese- and Platinum-Driven Oxidative and Nitrosative Stress in Oxaliplatin-Associated CIPN with Special Reference to Ca4Mn(DPDP)5, MnDPDP and DPDP

Platinum-containing chemotherapeutic drugs are efficacious in many forms of cancer but are dose-restricted by serious side effects, of which peripheral neuropathy induced by oxidative-nitrosative-stress-mediated chain reactions is most disturbing. Recently, hope has been raised regarding the catalytic antioxidants mangafodipir (MnDPDP) and calmangafodipir [Ca4Mn(DPDP)5; PledOx®], which by mimicking mitochondrial manganese superoxide dismutase (MnSOD) may be expected to overcome oxaliplatin-associated chemotherapy-induced peripheral neuropathy (CIPN). Unfortunately, two recent phase III studies (POLAR A and M trials) applying Ca4Mn(DPDP)5 in colorectal cancer (CRC) patients receiving multiple cycles of FOLFOX6 (5-FU + oxaliplatin) failed to demonstrate efficacy. Instead of an anticipated 50% reduction in the incidence of CIPN in patients co-treated with Ca4Mn(DPDP)5, a statistically significant increase of about 50% was seen. The current article deals with confusing differences between early and positive findings with MnDPDP in comparison to the recent findings with Ca4Mn(DPDP)5. The POLAR failure may also reveal important mechanisms behind oxaliplatin-associated CIPN itself. Thus, exacerbated neurotoxicity in patients receiving Ca4Mn(DPDP)5 may be explained by redox interactions between Pt2+ and Mn2+ and subtle oxidative-nitrosative chain reactions. In peripheral sensory nerves, Pt2+ presumably leads to oxidation of the Mn2+ from Ca4Mn(DPDP)5 as well as from Mn2+ in MnSOD and other endogenous sources. Thereafter, Mn3+ may be oxidized by peroxynitrite (ONOO-) into Mn4+, which drives site-specific nitration of tyrosine (Tyr) 34 in the MnSOD enzyme. Conformational changes of MnSOD then lead to the closure of the superoxide (O2•-) access channel. A similar metal-driven nitration of Tyr74 in cytochrome c will cause an irreversible disruption of electron transport. Altogether, these events may uncover important steps in the mechanism behind Pt2+-associated CIPN. There is little doubt that the efficacy of MnDPDP and its therapeutic improved counterpart Ca4Mn(DPDP)5 mainly depends on their MnSOD-mimetic activity when it comes to their potential use as rescue medicines during, e.g., acute myocardial infarction. However, pharmacokinetic considerations suggest that the efficacy of MnDPDP on Pt2+-associated neurotoxicity depends on another action of this drug. Electron paramagnetic resonance (EPR) studies have demonstrated that Pt2+ outcompetes Mn2+ and endogenous Zn2+ in binding to fodipir (DPDP), hence suggesting that the previously reported protective efficacy of MnDPDP against CIPN is a result of chelation and elimination of Pt2+ by DPDP, which in turn suggests that Mn2+ is unnecessary for efficacy when it comes to oxaliplatin-associated CIPN.

The Damaging Outcome of the POLAR Phase III Trials Was Due to Avoidable Time-Dependent Redox Interaction between Oxaliplatin and PledOx

On 2 July 2021, highly negative results were reported from the POLAR A and M phase III trials in patients with colorectal cancer, treated with an oxaliplatin-based regimen and co-treated with calmangafodipir (CaM; PledOx^®; PledPharma AB/Egetis Therapeutics AB) or placebo. The results revealed persistent chemotherapy-induced peripheral neuropathy (CIPN) in 54.8% of the patients treated with PledOx, compared with 40.0% of the patients treated with the placebo (p < 0.05), i.e., a 37% increase in incidence of the side effect that the trial was aimed to prevent. The damaging outcome of the trials differed diametrically from an in-parallel conducted mice study and from a clinical trial with mangafodipir, the active ingredient of CaM. According to the authors of the POLAR report, the etiology of the profound increase in CIPN in the PledOx arm is unclear. However, these devastating effects are presumably explained by intravenous administrations of PledOx and oxaliplatin being too close in time and, thereby, causing unfavorable redox interactions between Mn²⁺ and Pt²⁻. In the mice study as well as in the preceding phase II clinical trial (PLIANT), PledOx was administered 10 min before the start of the oxaliplatin infusion; this was clearly an administration procedure, where the devastating interactions between PledOx and oxaliplatin could be avoided. However, when it comes to the POLAR trials, PledOx was administered, for incomprehensible reasons, “on Top of Modified FOLFOX6” at day one, i.e., after the two-hour oxaliplatin infusion instead of before oxaliplatin. This is a time point when the plasma concentration of oxaliplatin and Pt²⁺-metabolites is at its highest, and where the risk of devastating redox interactions between PledOx and oxaliplatin, in turn, is at its highest.

May Mangafodipir or Other SOD Mimetics Contribute to Better Care in COVID-19 Patients?

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is characterized by massive inflammation of the arterial endothelium accompanied by vasoconstriction and widespread pulmonary micro thrombi. As a result, due to the destruction of nitric oxide (^•NO) by inflammatory superoxide (O₂^•-), pulmonary ^•NO concentration ceases, resulting in uncontrolled platelet aggregation and massive thrombosis, which kills the patients. Introducing ^•NO by inhalation (INO) may replace the loss of endothelium-derived ^•NO. The first results from clinical trials with INO in SARS-CoV-2 patients show a rapid and sustained improvement in cardiopulmonary function and decreased inflammation. An ongoing phase III study is expected to confirm the method's efficacy. INO may hence become a first line treatment in SARS-CoV-2 patients. However, due to the rapid inactivation of ^•NO by deoxyhemoglobin to nitrate, pulmonary administration of ^•NO will not protect remote organs. Another INO-related pharmacological approach to protect SARS-CoV-2 patients from developing life-threatening disease is to inhibit the O₂^•--driven destruction of ^•NO by neutralizing inflammatory O₂^•-. By making use of low molecular weight compounds that mimic the action of the enzyme manganese superoxide dismutase (MnSOD). The MnSOD mimetics of the so-called porphyrin type (e.g., AEOL 10150), salen type (e.g., EUK-8) and cyclic polyamine type (e.g., M40419, today known as GC4419 and avasopasem manganese) have all been shown to positively affect the inflammatory response in lung epithelial cells in preclinical models of chronic obstructive pulmonary disease. The Manganese diPyridoxyL EthylDiamine (MnPLED)-type mangafodipir (manganese dipyridoxyl diphosphate-MnDPDP), a magnetic resonance imaging (MRI) contrast agent that possesses MnSOD mimetic activity, has shown promising results in various forms of inflammation, in preclinical as well as clinical settings. Intravenously administration of mangafodipir will, in contrast to INO, reach remote organs and may hence become an important supplement to INO. From the authors' viewpoint, it appears logical to test mangafodipr in COVID-19 patients at risk of developing life-threatening SARS-CoV-2. Five days after submission of the current manuscript, Galera Pharmaceuticals Inc. announced the dosing of the first patient in a randomized, double-blind pilot phase II clinical trial with GC4419 for COVID-19. The study was first posted on ClinicalTrials.gov (Identifier: NCT04555096) 18 September 2020.

Manganese-Enhanced Magnetic Resonance Imaging: Overview and Central Nervous System Applications With a Focus on Neurodegeneration

Manganese-enhanced magnetic resonance imaging (MEMRI) rose to prominence in the 1990s as a sensitive approach to high contrast imaging. Following the discovery of manganese conductance through calcium-permeable channels, MEMRI applications expanded to include functional imaging in the central nervous system (CNS) and other body systems. MEMRI has since been employed in the investigation of physiology in many animal models and in humans. Here, we review historical perspectives that follow the evolution of applied MRI research into MEMRI with particular focus on its potential toxicity. Furthermore, we discuss the more current in vivo investigative uses of MEMRI in CNS investigations and the brief but decorated clinical usage of chelated manganese compound mangafodipir in humans.