The Role of Iron and Siderophores in Infection, and the Development of Siderophore Antibiotics

Abstract

Iron is an essential nutrient for bacterial growth, replication, and metabolism. Humans store iron bound to various proteins such as hemoglobin, haptoglobin, transferrin, ferritin, and lactoferrin, limiting the availability of free iron for pathogenic bacteria. However, bacteria have developed various mechanisms to sequester or scavenge iron from the host environment. Iron can be taken up by means of active transport systems that consist of bacterial small molecule siderophores, outer membrane siderophore receptors, the TonB-ExbBD energy-transducing proteins coupling the outer and the inner membranes, and inner membrane transporters. Some bacteria also express outer membrane receptors for iron-binding proteins of the host and extract iron directly from these for uptake. Ultimately, iron is acquired and transported into the bacterial cytoplasm. The siderophores are small molecules produced and released by nearly all bacterial species and are classified according to the chemical nature of their iron-chelating group (ie, catechol, hydroxamate, α-hydroxyl-carboxylate, or mixed types). Siderophore-conjugated antibiotics that exploit such iron-transport systems are under development for the treatment of infections caused by gram-negative bacteria. Despite demonstrating high in vitro potency against pathogenic multidrug-resistant bacteria, further development of several candidates had stopped due to apparent adaptive resistance during exposure, lack of consistent in vivo efficacy, or emergence of side effects in the host. However, cefiderocol, with an optimized structure, has advanced and has been investigated in phase 1 to 3 clinical trials. This article discusses the mechanisms implicated in iron uptake and the challenges associated with the design and utilization of siderophore-mimicking antibiotics.

Keywords: iron transport; monobactams; siderophore; siderophore-antibiotic conjugate; β-lactams.

Figure 1.

Iron transporters in gram-negative bacteria and metal availability in the host during infection. In a healthy individual, ferric iron (Fe³⁺; red circles) circulates bound by transferrin in the blood, and ferrous iron (Fe²⁺; green circles) is complexed in heme, which is bound by hemoglobin within red blood cells but can be released by hemolysis during infection. Free Fe²⁺ is uncommon; however, when available, it enters through the general porin pathway. Free heme is scavenged by hemopexin. Secreted bacterial siderophores remove iron from transferrins and ferritin, and the siderophore–iron complexes are bound by cognate receptors at the bacterial surface. Similarly, secreted hemophores such as HasA and HxuC can remove heme from hemoglobin and hemopexin. Enterobacteria also possess outer membrane receptors for heme. In Neisseria, iron transferrins are bound by outer membrane receptors comprising 2 subunits (eg, LbpA and LbpB for lactoferrin) and forced to release 1 of the bound iron ions. Catecholate-mediated iron acquisition (eg, by enterobactin) can be inhibited by the innate immune protein lipocalin-2 (siderocalin or neutrophil gelatinase-associated lipocalin), which binds and sequesters catechols.

Figure 2.

Structures of the monocyclic β-lactam-siderophore conjugates that have been evaluated for clinical development—BAL30072 (terminated in phase 1), MB-1 (preclinical investigation only), MC-1 (preclinical investigation only), and SMC-3176 (preclinical investigation only).

Figure 3.

Structures of the siderophore-conjugated cephalosporins that have been evaluated for clinical development. Compound 1: cefetecol (terminated in phase 1). Compound 2 (preclinical investigation only [73]): the halogen-substituted catechol moiety (highlighted in green), optimized for in vivo stability. Compound 3: GT-1 (currently in preclinical investigation). Compound 6: cefiderocol (phase 1 to phase 3 clinical development), which shares the halogen-substituted catechol moiety shown in compound 2. For comparison, structures are also shown for ceftazidime (compound 4), with which cefiderocol shares the bulky 7-acylamino side chain (highlighted in red) that confers β-lactamase stability, and cefepime (compound 5), with which cefiderocol shares the 3’ side chain with a quaternary ammonium function (highlighted in blue) that confers β-lactamase stability and good penetration into gram-negative bacteria.