Review pubs.acs.org/Organometallics
CO-Releasing Molecules: A Personal View Brian E. Mann* Department of Chemistry, University of Sheffield, Sheffield, United Kingdom S3 7HF ABSTRACT: The development of CO-releasing molecules, CO-RMs, for medical applications is reviewed from a personal point of view. The review covers the initial discovery of CORMs and then concentrates on developments involving the author. The review finishes with suggestions for areas meriting further investigation.
■
INTRODUCTION Carbon monoxide has a justified reputation of being a “silent killer” and causes many deaths every year. Nevertheless, it can be generated in every cell in mammals by both constitutive (HO-2) and inducible (HO-1) heme oxygenase enzymes.1 HO-1 catabolizes heme to biliverdin (1), CO, and FeII (see Scheme 1). The green biliverdin (1) is subsequently reduced to yellow bilirubin (2). The action of this enzyme is commonly observed
as a bruise develops. It starts off a red-purple with the liberated oxygenated heme. The heme oxygenase uses 3 mol of O2 to oxidize 1 mol of heme, which results in the bruise going anaerobic and generating the blue desoxyheme. Then the bruise turns yellow due to the bilirubin formed. The yellow of bilirubin is observed in urine and in jaundice. This enzyme is essential for mammalian life, since an individual who is born deficient in HO-1 will die prematurely.2
Scheme 1. Catabolism of Heme by Heme Oxygenase To Give FeII, CO, and Biliverdin
During the 1980s and 1990s it was discovered that NO is an important signaling molecule in mammals,3,4 which stimulated investigations of possible biological roles for other gaseous transmistters, including CO.5 Gradually the work on CO intensified, and it is now evident that this gas exerts multiple effects in mammals. It is anti-inflammatory, antiapoptotic, and antiproliferative, protects tissues against hypoxia or ischemiareperfusion injury, and causes vasodilatation. It has been shown to have a valuable role in preclinical animal models of cardiovascular disease, inflammatory disorders, and organ transplantation.5 This has led to several clinical trials.6−9 The medical use of CO gas presents problems. It is administered Special Issue: Organometallics in Biology and Medicine Received: May 1, 2012 Published: June 22, 2012 © 2012 American Chemical Society
5728
dx.doi.org/10.1021/om300364a | Organometallics 2012, 31, 5728−5735
Organometallics
Review
that the solution initially contained a ca. 40:60 mixture of 4 and 5. Subsequently, 5 isomerizes to 6.17 As [Ru2(CO)6Cl4] is commercially available, it has proved to be popular as a CO-RM for biological research. The initial work showed it to have low toxicity and to be a vasodilator.17 Approximately 150 papers have appeared so far on its biological use.
through the lungs as a mixture with air, typically 200 ppm. The whole body is exposed to it, and the level of carbonmonoxyhemoglobin has to be carefully monitored. It would be preferable to administer the CO only to selected parts of the body, and this possibility is offered by using a solution of a metal carbonyl whose quantity and location can be carefully controlled and also offers the possibility of using a high local dose. This has led to research into compounds which are stable enough to store yet release CO when introduced into the body. Several of these compounds have been used in animal models of cardiovascular disease, inflammatory disorders, and organ transplantation and reproduce many of the results obtained using CO gas.5 Recently, there have been two comprehensive reviews of CO-releasing molecules (CO-RMs) from a chemical point of view. In 2010 they were reviewed in Topics in Organometallic Chemistry,10 and there is a review due to appear in Comprehensive Inorganic Chemistry II.11 A recent medical review covers the applications of CO and CO-RMs in medicine.5 As a result of these recent reviews, no attempt is made in the present review to be comprehensive and it is written from a personal point of view about the development of CO-RMs.
■
DEVELOPMENT OF CO-RMS During the summer of 2000 a selection of potential CO-RMs were synthesized on the basis of known compounds. In view of [Ru2(CO)6Cl4] being a successful CO-RM, emphasis was placed on literature compounds and their analogues containing the RuII(CO)3 fragment. Three groups of ligands were used, amino acids, nucleotides, and nucleosides, because they are naturally present in the body and should not have toxicity problems.18 One of the simplest compounds, [Ru(CO)3Cl(glycinate)] (7; CORM-3) has proven to be a valuable CORM. The initial study showed that it has low toxicity, is a vasodilator, and greatly increased the survival of mice following heart transplants.19 This compound has become popular as a CO-RM to study the effect of CO on a wide range of biological systems, and approximately 100 papers have appeared so far on its use. Glycine is preferred over the other amino acids, as it does not contain a chiral center. There is a chiral center at the ruthenium, and the introduction of a second one results in diastereomers. This can be illustrated for [Ru(CO)3Cl(leucinate)], where the crystal structure shows the presence of both diastereomers in the unit cell (see Figure 1).20 The
■
DISCOVERY OF CO-RELEASING MOLECULES, CO-RMS Roberto Motterlini came to see me in Sheffield at the end of February 2000. Just by chance, our meeting followed an undergraduate tutorial which had included isoelectronic compounds and electron counts and I had on my whiteboard NO+ ≡ CO
This was particularly fortuitous. I knew that he was coming to see me about making some compounds but did not know that his interest was CO release and its use as a signaling molecule. The role of NO in vivo as a signaling molecule had been established during the 1980s,12 and a number of NOreleasing molecules are being used in medical applications.13−15 Roberto briefed me on the heme oxygenase enzyme system and the beneficial role of CO in vivo, especially in the cardiovascular system.16 He was interested in using metal carbonyls to deliver carbon monoxide. He had already tried three commercially available metal carbonyls, [Mn2(CO)10] (CORM-1), [Fe(CO)5], and [Ru2(CO)6Cl4] (CORM-2).17 The initial results were very encouraging. They caused sustained vasodilation in precontracted rat aortic rings, attenuated coronary vasoconstriction in hearts ex vivo, and significantly reduced acute hypertension in vivo. All these carbonyls had problems. [Mn2(CO)10] (CORM-1) and [Fe(CO)5] are insoluble in water and require photolysis to release CO. [Ru2(CO)6Cl4] (CORM-2) is also insoluble in water but can be used as a solution in DMSO; however, it only releases to myoglobin around 0.9 mol of CO/mol of [Ru2(CO)6Cl4]. He asked if I was willing to collaborate on the development of water-soluble CO-RMs. I readily agreed, and this was the beginning of a very productive research collaboration. As [Mn2(CO)10] and [Fe(CO)5] require photolysis and are insoluble in water and [Fe(CO)5] is highly toxic, it was decided to look for other metal carbonyls which are water-soluble, release CO readily, and would be easier to use in biological applications. The first step was to establish what happens to [Ru2(CO)6Cl4] when it is dissolved in DMSO and then synthesize other possible CO-RMs. A detailed 1H and 13C NMR investigation of [Ru2(CO)6Cl4] (3) in DMSO showed
Figure 1. Crystal structure of [Ru(CO)3Cl(leucinate)].20
other CO-RMs containing the RuII(CO)3 fragment prepared by the summer students were tested for CO release and appeared to offer no advantage over [Ru(CO)3Cl(glycinate)] and have not been investigated further.
Most of the other compounds made by the summer students were based on FeII carbonyls. Several of the iron compounds also looked promising. [(η-C5H5)Fe(CO)3]+ was found to release CO very rapidly but precipitated after CO loss, raising 5729
dx.doi.org/10.1021/om300364a | Organometallics 2012, 31, 5728−5735
Organometallics
Review
Scheme 2. Summary of the Most Probable Species Formed in Equilibria Involving [Ru(CO)3Cl(glycinate)] with Base
Scheme 3. Effect of Dilute Hydrochloric Acid on an Aqueous Solution of [Ru(CO)3Cl(glycinate)]
(CO)2 “impurity” is [Ru(CO)2(CO2H)Cl(glycinate)]−, which is formed at ca. pH 3 in water.22 Further investigation showed that it has a rich aqueous chemistry (see Scheme 2). It reacts with water; the first step liberates a proton, which lowers the pH. When the pH is increased, there are two further equilibria, probably arising from the deprotonation of the RuCO2H group and replacement of Ru−Cl by Ru−OH.22 Acidification of [Ru(CO)3Cl(glycinate)] first results in the glycinate becoming monodentate with protonation of the carboxylic group, followed by loss of the glycine (see Scheme 3). As a result of these equilibria, it is difficult to obtain pure [Ru(CO)3Cl(glycinate)]. The possibility of other metal carbonyls being viable was then examined. We were presented with the problem of deciding criteria to select CO-RMs. CO release to myoglobin provides an easy way to quantify the CO lability of the CO-RM, but as we will see later, it is not without problems. The biological test chosen was the effect on macrophages, testing cell viability, cell toxicity, and reduction of nitrite production after treatment of the cells with lipopolysaccharide. Deciding what is an acceptable half-life for CO release to myoglobin is subjective, but it was arbitrarily decided that less than 100 min was preferred. Subsequent Development of CO-RMs. In the initial screening of compounds, [(η-C5H5)Fe(CO)3]+ had looked interesting, but after CO release precipitation occurred, which could block microarteries.18 In order to get around this problem, substituents were introduced into the cyclopentadienyl group. The introduction of a −CO2Me group with [(ηC5H4CO2Me)Fe(CO)3]+ opens up transesterification to permit variation in water and lipid solubility.23 The −CO2Me group also increases the lability of the CO, and the half-life for CO release decreased from 200 min for [(η-C5H5)Fe(CO)2Br] to 38 min for [(η-C5H4CO2Me)Fe(CO)2Br]. [(η-indenyl)Fe(CO)3]+ and its derivatives were also examined, but there were solubility problems and they appeared to offer no advantages over [(η-C5H5)Fe(CO)3]+ and its derivatives.24 [(η-C5H4CO2Me)Fe(CO)3]+ and [(η-C5H4CO2CH2CH2OH)Fe(CO)3]+ looked promising, but a more interesting group of manganese carbonyls was discovered, and interest has switched to them.
worries about blocking microarteries. 1 8 [Fe(CO) 2 (H2NCH2CH2NH2)(SPh)2] and [Fe(CO)2(2,2′-bpy)(SPh)2] cause vasodilatation.18 The compounds synthesized during this initial investigation were used as examples in a patent to cover the use of metal carbonyls as CO-RMs for medical applications.18 This provided a platform to form a company, Hemocorm, and provide a way to access venture capital to help to fund the development of CO-RMs and test their potential in medicine. The commercial aspect of the work did produce the problem that work could not be published until it was patented. The rate of CO release is mainly influenced by the atoms forming the first and, to a limited extent, the second coordination sphere, while solubility and toxicity are determined by the whole molecule. Hence, the first and second coordination spheres of the metal became commercially sensitive but the class of compounds could not be patented until the problem of toxicity and solubility was solved. Thus, CO-RMs are not like most pharmaceuticals that interact with a receptor, where the potency is determined by the periphery of the compound. As a result, many compounds have been synthesized and tested as CO-RMs but not reported, even though they release CO. [Ru(CO)3Cl(glycinate)] (CORM-3, 7). The chemical research continued following two themes: to understand the chemistry of [Ru(CO)3Cl(glycinate)] and to develop more CO-RMs. At first sight, [Ru(CO)3Cl(glycinate)] looks to be a simple compound and has excellent biological properties. It has low toxicity, and
