Review Cite This: ACS Infect. Dis. XXXX, XXX, XXX−XXX pubs.acs.org/journal/aidcbc Signed, Sealed, Delivered: Conjugate and Prodrug Strategies as Targeted Delivery Vectors for Antibiotics † † ‡ Ana V. Cheng and William M. Wuest*, , † Department of Chemistry, Emory University, 1515 Dickey Drive, Atlanta, Georgia 30322, United States ‡ Emory Antibiotic Resistance Center, Emory School of Medicine, 201 Dowman Drive, Atlanta, Georgia 30322, United States ABSTRACT: Innate and developed resistance mechanisms of bacteria to antibiotics are obstacles in the design of novel drugs. However, antibacterial prodrugs and conjugates have shown promise in circumventing resistance and tolerance mechanisms via directed delivery of antibiotics to the site of infection or to specific species or strains of bacteria. The selective targeting and increased permeability and accumu- lation of these prodrugs not only improves efficacy over unmodified drugs but also reduces off-target effects, toxicity, and development of resistance. Herein, we discuss some of these methods, including sideromycins, antibody-directed prodrugs, cell penetrating peptide conjugates, and codrugs. KEYWORDS: oligopeptide, sideromycin, antibody−antibiotic conjugate, cell penetrating peptide, dendrimer, transferrin inding new and innovative methods to treat bacterial F infections comes with many inherent challenges in addition to those presented by the evolution of resistance mechanisms. The ideal antibiotic is nontoxic to host cells, permeates bacterial cells easily, and accumulates at the site of infection at high concentrations. Narrow spectrum drugs are also advantageous, as they can limit resistance development and leave the host commensal microbiome undisturbed.1 However, various resistance mechanisms make pathogenic infections difficult to eradicate: Many bacteria respond to antibiotic pressure by decreasing expression of active transporters and porins2 and 3,4 Downloaded via EMORY UNIV on April 18, 2019 at 12:34:17 (UTC). increasing expression of efflux pumps, making it difficult for drugs to infiltrate cells and achieve killing concentrations. Furthermore, several species of bacteria sequester themselves in human macrophages5 or in biofilms,6 leading to persistent ffi See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles. infections, which are di cult to reach with traditional antimicrobials. These problems can be complicated or even impossible to address with simple small molecules. An emerging strategy to overcome these challenges is the employment of antibiotic conjugates. In this Review, we will focus on the application of sophisticated conjugate strategies to enhance antibacterial activity. The conjugates covered enable temporary masking of activity through a cleavable prodrug linkage, directed delivery of a drug through conjugation, improved pharmacoki- netic and pharmacodynamic (PK/PD) profiles, or some combination of the three. Figure 1. Standard prodrugs. ■ STANDARD PRODRUGS revealed as precursor compounds, such as phenacetin (the By definition, a prodrug is an inactive compound that undergoes precursor to acetaminophen) and chloral hydrate (which is a chemical transformation in vivo to remove a covalently linked moiety and release the active form of the drug. Adrien Albert Received: January 15, 2019 coined the term in 1958 in reference to drugs that had been © XXXX American Chemical Society A DOI: 10.1021/acsinfecdis.9b00019 ACS Infect. Dis. XXXX, XXX, XXX−XXX ACS Infectious Diseases Review Figure 2. Codrugs. Sites of cleavage are marked in red. converted by alcohol dehydrogenase to trichloroethanol, a drug to its eventual FDA approval in 2014.18 Other strategies for sedative) (Figure 1).7 Throughout history, several drugs have increased aqueous solubility include the addition of an ionizable been discovered in their prodrug form and were only later amine,21,22 succinic acid,23 sugars,24 and polyethylene glycol25 realized to be a precursor to the active compound. For instance, and the transformation of sulfides to sulfoxides.26 These groups Gerhard Domagk won a Nobel prize for the development of yield the active compounds upon O → N acyl migration,21 Prontosil (Figure 1), one of the earliest modern antibiotics.8 enzymatic ester23,27 or glycosidic hydrolysis,24 proteolytic However, it was later identified as a precursor to the actual active cleavage,28 or reduction.26 Conversely, when the lipophilicity compound, sulfanilamide, likely metabolized by azoreductases of a drug must be increased for improved passive permeability, produced either in the liver or by gut microbiota.9 Today, − hydrophobic promoieties that can be hydrolyzed by esterases or prodrugs are used for the treatment of cancer,10 12 neuro- peptidases may be appended to polar or ionized groups.29,30 logical13,14 and cardiovascular diseases,15,16 and bacterial Commonly, alkyl and aryl esters15,31 or N-acylated groups32 are infections. In the past decade, the US Food and Drug synthesized to this effect. These moieties can also aid in Administration (FDA) has approved 30 prodrugs, 10% of 17 − improving the metabolic stability of a drug, prolonging its which are for antibacterial applications.17 19 duration of action,33 or transport across the blood−brain Prodrug strategies have traditionally been applied to rescue barrier.34 20 prospective drugs with disfavorable PK/PD profiles; the The principles of simple prodrug design described above have interplay between the steric and electronic influences of a prompted the development of much more sophisticated chemical structure and its PK/PD profiles is not always antibiotic conjugates. In fact, many of the approaches described straightforward, so designing improved drug derivatives often hereafter take advantage of similar mechanisms of linker becomes a juggling act guided by the chemical intuition of cleavage (highlighted throughout): enzyme- or environment- medicinal chemists. Additionally, the optimization process can promoted drug release proves crucial to the success of many involve years of modification after each round of in vitro or in conjugates. vivo results. Prodrugs allow chemists to install a temporary moiety in a molecule, removable by enzymes or other ■ CODRUGS environmental conditions, to address one of the PK/PD 35 variables without affecting the activity and binding of the active Sometimes called mutual prodrugs, codrugs consist of two compound, saving time and resources in the drug development covalently linked entities whose cleavage releases two different process. active drugs. Codrugs benefit from improved PK/PD profiles in Depending on the active molecule under investigation, it may the same way that simple prodrugs do. Choosing the site of be desirable to increase hydrophilicity or lipophilicity to linkage on each compound carefully can mask labile groups, improve solubility or passive permeability, respectively. The which are otherwise prone to degradation. They also enjoy promoieties used in these situations tend to be small, simple in advantages over joint administration of separate therapies. For function, and relatively easy to introduce synthetically. For instance, the type of linker allows control over the site of release example, the addition of phosphate groups has greatly improved and can confer additional metabolic stability. The intertwined the aqueous solubility of several compounds; two of the three PK/PD properties of the linked drugs ensure the equal dosing of antibacterial prodrugs approved by the FDA in the past decade the two entities for full utilization of their tandem or synergistic are phosphate esters. Ceftaroline fosamil (Figure 1)isanN- functions. Codrugs are an intriguing innovation, especially as phosphono prodrug of the novel cefozopran derivative T-91825 combination therapies gain popularity. In addition to the for the treatment of methicillin-resistant Staphylococcus aureus benefits listed above, they have the potential to limit resistance (MRSA).17 The addition of an N-phosphono group, cleaved in development by inhibiting multiple targets. Unfortunately, their the body by plasma phosphatases, boosted the solubility of their application is limited to compatible therapies, which benefit lead compound from 2.3 mg/mL to >100 mg/mL. Similarly, from release in the same environment. Additionally, they are addition of a phosphate to tedizolid (Figure 1), an oxazolidinone mostly limited to 1:1 combinations, as it can become difficult to antibiotic for the treatment of several Gram-positive infections, rationally link more than two drug units. Consequently, this improved water solubility and facilitated the advancement of the limits combinations to drugs with similar potencies. There are B DOI: 10.1021/acsinfecdis.9b00019 ACS Infect. Dis. XXXX, XXX, XXX−XXX ACS Infectious Diseases Review limited examples of this technique, making codrugs a prime area membranes and resultant internalization. Unfortunately, hydro- for development. lytic drug release yielded squalenic acid as a byproduct, leading Codrugs were first introduced in 1980 with the development to high toxicity. Abed et al. hypothesized that shorter terpenes of a β-lactam/β-lactamase inhibitor (ampicillin/sulbactam) would limit cytotoxicity and instead synthesized acid labile codrug, sultamicillin (Figure 2).36 Ampicillin and sulbactam geranyl- and farnesyl-penicillin G (Figure 3).43 Indeed, the are poorly absorbed when administered separately, but geranyl-penG prodrug showed decreased toxicity in murine μ sultamicillin displayed fast