Downloaded from http://perspectivesinmedicine.cshlp.org/ on September 30, 2021 - Published by Cold Spring Harbor Laboratory Press Pleuromutilins: Potent Drugs for Resistant Bugs—Mode of Action and Resistance Susanne Paukner and Rosemarie Riedl Nabriva Therapeutics AG, A-1110 Vienna, Austria Correspondence: [email protected]; [email protected] Pleuromutilins are antibiotics that selectively inhibit bacterial translation and are semisyn- thetic derivatives of the naturally occurring tricyclic diterpenoid pleuromutilin, which re- ceived its name from the pleuromutilin-producing fungus Pleurotus mutilus. Tiamulin and valnemulin are two established derivatives in veterinary medicine for oral and intramuscular administration. As these early pleuromutilin drugs were developed at a time when companies focused on major antibacterial classes, such as the b-lactams, and resistance was not regard- ed as an issue, interest in antibiotic research including pleuromutilins was limited. Over the last decade or so, there has been a resurgence in interest to develop this class for human use. This has resulted in a topical derivative, retapamulin, and additional derivatives in clinical development. The most advanced compound is lefamulin, which is in late-stage develop- ment for the intravenous and oral treatment of community-acquired bacterial pneumonia and acute bacterial skin infections. Overall, pleuromutilins and, in particular, lefamulin are characterized by potent activity against Gram-positive and fastidious Gram-negative patho- gens as well as against mycoplasmas and intracellular organisms, such as Chlamydia spp. and Legionella pneumophila. Pleuromutilins are unaffected by resistance to other major antibiotic classes, such as macrolides, fluoroquinolones, tetracyclines, b-lactam antibiotics, and others. Furthermore, pleuromutilins display very low spontaneous mutation frequencies and slow, stepwise resistance development at sub-MIC in vitro. The potential for resistance development in clinic is predicted to be slow as confirmed by extremely low resistance rates to this class despite the use of pleuromutilins in veterinary medicine for .30 years. Although rare, resistant strains have been identified in human- and livestock-associated environments and as with any antibiotic class, require close monitoring as well as prudent use in veterinary medicine. This review focuses on the structural characteristics, mode of action, antibacterial www.perspectivesinmedicine.org activity, and resistance development of this potent and novel antibacterial class for systemic use in humans. leuromutilins are a well-known class of an- (Kavanagh et al. 1951). Semisynthetic derivati- Ptibiotics discovered in the 1950s by the iso- zations have led to tiamulin and valnemulin, lation of the naturally occurring pleuromutilin which were introduced to veterinary medicine from Pleurotus mutilus (now renamed Clitophi- in 1979 and 1999, respectively, for the treatment lus scyphoides), an edible mushroom (Fig. 1) of pulmonary and intestinal infections caused Editors: Lynn L. Silver and Karen Bush Additional Perspectives on Antibiotics and Antibiotic Resistance available at www.perspectivesinmedicine.org Copyright # 2017 Cold Spring Harbor Laboratory Press; all rights reserved; doi: 10.1101/cshperspect.a027110 Cite this article as Cold Spring Harb Perspect Med 2017;7:a027110 1 Downloaded from http://perspectivesinmedicine.cshlp.org/ on September 30, 2021 - Published by Cold Spring Harbor Laboratory Press S. Paukner and R. Riedl 20 18 19 OH O 13 12 11 HO 10 17 21 O 14 22 H 15 5 4 9 16 1 Pleurotus mutilus 7 (Clitopilus scyphoides) 6 8 3 O 2 Figure 1. Structure of pleuromutilin with numbering system (Arigoni 1968). (Left panel from Lindsey 2006.) by Mycoplasma spp., Brachyspira spp., and Law- development for topical and oral administra- sonia intracellularis in pigs, poultry, and, to tion, respectively. In addition, recent research some extent, in rabbits. Despite the use of pleu- has been directed at further extending the romutilins for treatment in veterinary medicine antibacterial spectrum to include the ESKAPE for more than three decades, resistance devel- pathogens (Boucher et al. 2009; Paukner et opment has been uncommon. This can likely al. 2014a,b, 2015a,b,c; Strickmann et al. 2014; be attributed to several factors including the Wicha and Ivezic-Schoenfeld 2014; Wicha et al. unique and highly specific mode of action of 2015b). the pleuromutilins. Further, this class has not To the question “Are pleuromutilins finally been used for enhancement of food-producing fit for human use?” (Novak 2011), which has animal production (e.g., as growth promoters been raised because of unjustified anecdotal or for enhancement of feed efficiency) unlike concerns regarding metabolic stability, gastro- the tetracyclines, penicillins, or sulfonamides intestinal side effects, cardiac safety, or intrave- (EMA 2014a,b). Even though oral valnemulin nous tolerability, a clear response can be given: has been reported to be efficacious in the treat- Yes. In a phase 2 study, lefamulin was well- ment of persistent or life-threatening mycoplas- tolerated and showed comparable efficacy to ma infection in humans (Heilmann et al. 2001), IV vancomycin in patients with ABSSSI (Prince no pleuromutilin had received marketing au- et al. 2013). Despite challenging medicinal thorization by the end of the last century. chemistry, a number of compounds are in the In the new millennium, interest in the pleu- pipeline and further developments in this anti- www.perspectivesinmedicine.org romutilin class significantly increased as evi- biotic class are anticipated. denced by the development of new derivatives for human use. Retapamulin, a topical agent, was the first to be approved for the treatment PLEUROMUTILINS—MODE OF ACTION, of impetigo and infected small lacerations, ACTIVITY, AND RESISTANCE abrasion or sutured wounds caused by Staphy- Structure lococcus aureus and Streptococcus pyogenes (FDA 2007; EMA 2008). More recently, lefamulin, the The diterpenoid pleuromutilin comprises a tri- first pleuromutilin for intravenous and oral cyclic scaffold with unique annelation of a five-, administration, has entered into late-stage clin- six-, and eight-membered ring and eight stable ical development for the treatment of commu- chiral centers, as well as a glycolic ester moiety nity-acquired bacterial pneumonia (CABP) and forming the side chain also regarded as an ex- acute bacterial skin and skin structure infec- tension at position C14 (Fig. 1) (Anchel 1952; tions (ABSSSIs). BC-7013 and BC-3205, which Arigoni 1962, 1968; Birch et al. 1963, 1966). have a similar antibacterial profile but differ in Remarkable efforts have been made to achieve ADMET properties, are in early-stage clinical chemical modifications at several positions of 2 Cite this article as Cold Spring Harb Perspect Med 2017;7:a027110 Downloaded from http://perspectivesinmedicine.cshlp.org/ on September 30, 2021 - Published by Cold Spring Harbor Laboratory Press Pleuromutilins: Potent Drugs for Resistant Bugs the tricyclic core (Naegeli 1961; Berner et al. characteristics, including exceptional solubil- 1980, 1981, 1983, 1987; Brooks and Hunt 2000; ity, potent antimicrobial activity, and excellent Bacque´ et al. 2002; Springer et al. 2003, 2008; ADMET properties including metabolic stabil- Takadoi et al. 2007; Wang et al. 2009; Paukner ity enabling administration by both the intra- et al. 2015b), as well as biotransformation (Han- venous and oral routes. son et al. 2002), and the total synthesis of this unique scaffold (Gibbons 1982; Paquette and Mode of Action Bullman-Page 1985; Paquette and Wiedemann 1985; Bacque´ et al. 2003; Liu et al. 2011; Lotesta Pleuromutilins inhibit bacterial protein synthe- et al. 2011; Ruscoe et al. 2015). Most modifi- sis by binding to the central part of domain V cations, however, are primarily performed at of the 50S ribosomal subunit at the peptidyl the glycolic side chain of pleuromutilin with transferase center (PTC), which prevents the replacement of the terminal hydroxyl group or correct positioning of the CCA ends of tRNAs the entire side chain resulting in semisynthetic for peptide transfer in the A- and P-site, thereby pleuromutilin analogs of twomain types: (1) the inhibiting peptide bond formation (Hogenauer flexible sulfanylacetyl, and (2) the rigid acyl- 1975; Hogenauer and Ruf 1981; Hogenauer carbamate linker type. Despite significant ef- et al. 1981; Poulsen et al. 2001; Schlunzen et al. forts in the field of acylcarbamate pleuromuti- 2004; Long et al. 2006; Davidovich et al. 2007). lins by the GlaxoSmithKline group (Hunt 2000; Figure 3 shows the positioning of lefamulin in Brooks et al. 2001; Andemichael et al. 2009), the PTC of the bacterial ribosome in relation to only sulfanylacetyl derivatives, mostly with one the positions of A- and P-site tRNA. Positioning basic function at the side chain, have progressed is similar for various pleuromutilin derivatives beyond phase 1 clinical studies. The early work in that the tricyclic core is located in a pocket of the Sandoz group resulted in lipophilic orally close to the A-site, whereas the C14 side chain available veterinary products, tiamulin (Egger extends toward the P-site hindering the 30-end and Reinshagen 1978) and valnemulin (Fig. tRNA A- to P-site rotary motion, as shown by 2A) (Berner and Vyplel 1987), whereas work various footprinting and crystallographic stud- from GlaxoSmithKline and Sandoz/Nabriva ies for tiamulin, valnemulin (Poulsen