Macrocyclic natural products have evolved to fulfil numerous biochemical functions, and their profound pharmacological properties have led to their development as drugs. A macrocycle provides diverse functionality and stereochemical complexity in a conformationally pre-organized ring structure. This can result in high affinity and selectivity for protein targets, while preserving sufficient bioavailability to reach intracellular locations. Despite these valuable characteristics, and the proven success of more than 100 marketed macrocycle drugs derived from natural products, this structural class has been poorly explored within drug discovery. This is in part due to concerns about synthetic intractability and non-drug-like properties. This Review describes the growing body of data in favour of macrocyclic therapeutics, and demonstrates that this class of compounds can be both fully drug-like in its properties and readily prepared owing to recent advances in synthetic medicinal chemistry.

The first detection and characterization of oxomanganese(V) porphyrin complexes under ambient catalytic conditions is described. The reaction of (tetra-(N-methylpyridyl)porphyrinato)manganese(III) [Mn(III)TMPyP] with a variety of oxidants such as m-chloroperoxybenzoic acid (m-CPBA), HSO5 -, and ClO- has been shown to produce the same, short-lived intermediate (1) by stopped-flow spectrophotometry. The Soret maximum of 1 was found at 443 nm, intermediate between that of oxomanganese(IV) (428 nm) and Mn(III)TMPyP (462 nm), thus facilitating its detection. The rate of formation of 1 from Mn(III)TMPyP followed second-order kinetics, first order in Mn(III) porphyrin and first order in oxidant. The rate constants have the following order:  m-CPBA (2.7 × 107 M-1 s-1) > HSO5 - (6.9 × 105 M-1 s-1) ≈ ClO- (6.3 × 105 M-1 s-1). Once formed, the intermediate species 1 was rapidly converted to oxoMn(IV) (2) by one-electron reduction with a first-order rate constant of 5.7 s-1. The oxoMn(IV) species 2 was relatively stable under the reaction conditions, decaying slowly to Mn(III)TMPyP with a first-order rate constant of 0.027 s-1. The identity of 1 as an oxomanganese(V) complex was indicated by its reactivity. The one-electron reduction of 1 to oxoMn(IV) was greatly accelerated by nitrite ion (k = 1.5 × 107 M-1 s-1). However, the reaction between nitrite and oxoMn(IV) is much slower (k = 1.4 × 102 M-1 s-1). The oxoMn(V) intermediate 1 was shown to be highly reactive toward olefins, affording epoxide products. By contrast, oxoMn(IV) (2) was not capable of effecting the same reaction under these conditions. In the presence of carbamazepine (3) efficient oxygen transfer from the highly reactive oxoMn(V) (1) to the olefin (second-order rate constant of 6.5 × 105 M-1 s-1) resulted in the conversion of 1 directly back to Mn(III)TMPyP without the appearance of the stable oxoMn(IV) intermediate 2. With m-CPBA as the oxidant in the presence of H2 18O, the product epoxide was shown to contain 35% 18O, consistent with an O-exchange-labile oxoMn(V) intermediate. Nitrite ion inhibited the epoxidation reaction competitively by one electron reduction of the oxoMn(V) intermediate to the unreactive oxoMn(IV).

Peroxynitrite (ONOO ؊ ) is a potent oxidant implicated in a number of pathophysiological processes. The activity of ONOO ؊ is related to its accessibility to biological targets before its spontaneous decomposition (t 1͞2 Ϸ 1 s at pH 7.4, 37°C). Using model phospholipid vesicular systems and manganese porphyrins as reporter molecules, we demonstrated that ONOO ؊ freely crosses phospholipid membranes. The calculated permeability coefficient for ONOO ؊ is Ϸ8.0 ؋ 10 ؊4 cm⅐s ؊1 , which compares well with that of water and is Ϸ400 times greater than that of superoxide. We suggest that ONOO ؊ is a significant biological effector molecule not only because of its reactivity but also because of its high diffusibility. Peroxynitrite (ONOO Ϫ) has emerged as an important member of the family of reactive oxygen and nitrogen species (1-5) since the recognition of its rapid formation from nitric oxide (NO Ϫ⅐ ) and superoxide anion (O 2 Ϫ⅐ ) (6) The production of ONOO Ϫ in vivo has been demonstrated in the macrophage immune response (7,8) and under conditions of oxidative stress such as ischemia͞reperfusion (9-11). The reactions of ONOO Ϫ with biological substrates are known to include the nitration of tyrosine residues in proteins (12) and the oxidation of redox metal centers (13,14), DNA (15, 16), lipids (17), sulfhydryls (18), and methionine (19). In light of this reactivity, ONOO Ϫ has been implicated in a number of pathological conditions including neurological disorders (20-23) such as Alzheimer disease and amyotrophic lateral sclerosis, in atherosclerosis (24, 25), and a variety of conditions precipitated by endothelial injury (26). Furthermore, since nitration of tyrosine has been shown to block tyrosine phosphorylation, a key event in signal transduction cascades, the role of ONOO Ϫ as a signal molecule has been under investigation (27,28). It also has been demonstrated that ONOO Ϫ nitrates and inactivates manganese superoxide dismutase in chronic rejection of human renal allografts, which was proposed to be a general mechanism for the amplification of ONOO Ϫ oxidative damage (29). Given the short lifetime of ONOO Ϫ (t 1͞2 Ϸ 1 s at pH 7.4, 37°C) (30), a diffusion distance of Ϸ100 m has been estimated in physiological buffers (31). However, cells are compartmentalized into membrane-protected organelles (32). Thus, an important determinant of toxicity or signaling effectiveness of ONOO Ϫ will lie in its invasiveness and ability to access biological targets. Here, we demonstrate that ONOO Ϫ can diffuse freely across phospholipid membrane bilayers to react with target substrates. Thus, the significance of ONOO Ϫ as a biological effector molecule will derive not only from its reactivity but also its diffusibility. Vesicles Preparation. Small unilamellar vesicles (SUV) were prepared by using the method of ultrasonication of DMPC thin films containing Mn(III)ChP (35). The porphyrin was replaced by retinoic acid (40 M) or tocopherol (80 M) in the lipid thin films for SUV used in the Fe(III)TMPyP protection experiments. ...

Superoxide (O2 •-) and peroxynitrite (ONOO-) have been implicated in many pathophysiological conditions. To develop novel catalysts that have both ONOO- decomposition and O2 •- dismutase activity, and to understand the mechanisms of these processes, we have explored the reactivity of 5,10,15,20-tetrakis(N-methyl-4‘-pyridyl)porphinatomanganese(III) [Mn(III)TMPyP] toward ONOO- and O2 •-. The reaction of Mn(III)TMPyP with ONOO- to generate an oxomanganese(IV) porphyrin species [(oxoMn(IV)] is fast, but Mn(III)TMPyP is not catalytic for ONOO- decomposition because of the slow reduction of oxoMn(IV) back to the Mn(III) oxidation state. However, biological antioxidants such as ascorbate, glutathione, and Trolox rapidly turn over the catalytic cycle by reducing oxoMn(IV). Thus, Mn(III)TMPyP becomes an efficient peroxynitrite reductase when coupled with ascorbate, glutathione, and Trolox (k c ≈ 2 × 106 M-1 s-1), though the direct reactions of ONOO- with these biological antioxidants are slow (88 M-1 s-1, 5.8 × 102 M-1 s-1, and 33 M-1 s-1, respectively). Mn(III)TMPyP is known to catalyze the dismutation of O2 •-, and using stopped-flow spectrophotometry, the rate of Mn(III)TMPyP-catalyzed dismutation has been measured directly (k c = 1.1 × 107 M-1 s-1). Further, O2 •-, like the biological antioxidants, rapidly reduces oxoMn(IV) to the Mn(III) oxidation state (k ≈ 108 M-1 s-1), transforming Mn(III)TMPyP into a O2 •--coupled ONOO- reductase. Under conditions of oxidative stress and reduced antioxidant levels, Mn(III)TMPyP may deplete O2 •- primarily as a function of its ONOO- reductase activity, and not through its O2 •- dismutase activity.

Peroxynitrite (ONOO-) is a major cytotoxic agent that has been implicated in a host of pathophysiological conditions; it is therefore important to develop therapeutic agents to detoxify this potent biological oxidant, and to understand the modes of action of these agents. Water-soluble iron porphyrins, such as 5,10,15,20-tetrakis(N-methyl-4‘-pyridyl)porphinatoiron(III) [Fe(III)TMPyP] and 5,10,15,20-tetrakis(2,4,6-trimethyl-3,5-sulfonatophenyl)porphinatoiron(III) [Fe(III)TMPS], have been shown to catalyze the efficient decomposition of ONOO- to NO3 - and NO2 - under physiological conditions. However, the mechanisms of ONOO- decomposition catalyzed by these water-soluble iron porphyrins have not yet been elucidated. We have shown that there are two different pathways operating in the catalytic decomposition of ONOO- by FeTMPyP. Fe(III)TMPyP reacts rapidly with ONOO- to produce oxoFe(IV)TMPyP and NO2 (k ≈ 5 × 107 M-1 s-1). The oxoFe(IV) porphyrin, which persisted throughout the catalytic decomposition of ONOO-, was shown to be relatively unreactive toward NO2 and NO2 -. This oxoFe(IV) porphyrin was also shown to react with ONOO- (k = 1.8 × 106 M-1 s-1), and it was this oxoFe(IV)-ONOO- reaction pathway that predominated under conditions of excess ONOO- with respect to Fe(III)TMPyP. The competition between the two pathways explains the highly nonlinear relationship observed for k cat with respect to ONOO- concentration. Fe(III)TMPyP is also known to catalyze the dismutation of the ONOO- precursor superoxide (O2 -•), and using stopped-flow spectrophotometry, the rate of Fe(III)TMPyP-catalyzed O2 -• dismutation has been determined to be 1.9 × 107 M-1 s-1 by direct measurement. A detailed mechanistic understanding of how iron porphyrins function in the catalytic decomposition of both ONOO- and O2 -• may prove essential in the exploration of the chemistry and biology of these reactive oxygen species, and in understanding the biological activity of these metalloporphyrins.