bDrug resistance has become a global threat that, if not addressed, may return us to the preantibiotic era. A way to overcome the problem of growing incidence of global antibiotic resistance is to introduce compounds belonging to classes that are new to the clinic. During a screening of the marine microbial extract library for new antibiotics, one of the extracts showed promising antibacterial activity against Gram-positive organisms. Bioactivity-guided isolation and characterization of active metabolites led to the discovery of a novel thiazolyl cyclic-peptide antibiotic, PM181104. It was isolated and characterized from a marine spongeassociated actinobacterium strain of the genus Kocuria (MTCC 5269). The compound exhibited a potent in vitro antibacterial activity against a broad range of Gram-positive bacteria, including methicillin-resistant Staphylococcus aureus (MRSA) and vancomycin-resistant enterococci (VRE). The MIC values evaluated for the compound were found to be in the single-digit nanomolar range. In in vivo studies of PM181104 in a BALB/c murine septicemia model, the compound displayed 100% effective dose (ED 100 ) values of 2.5 and 5.0 mg/kg of body weight against MRSA and 10.0 mg/kg against VRE. In this report, in vitro and in vivo studies of PM181104 are described.

Thiazolyl cyclic peptide antibiotics are known for their poor aqueous solubility and unfavorable pharmacokinetics (PK) and hence pose challenging tasks in developing these antibiotics as clinical candidates. In the current paper, we report a possible way to address these challenges with exemplification of our antibiotic PM181104. The approach was to prepare formulations with known excipients, Polysorbate 80 (Tween 80, T-80) and PEG 400 through their varied stiochiometric combination in appropriate ratio to achieve acceptable osmolarity, pH and particle size of the formulation. Two different sets of formulations were prepared with two distinct average particle diameters ranging from 32.8 to 465.4 nm. First, semi-transparent solutions with a particle size of >100 nm were achieved by keeping concentration of PEG 400 constant at 8% (w/v) and decreasing the amounts of T-80. Second, clear colorless solutions with a particle size of <100 nm were achieved by keeping concentration of T-80 constant at 8% (w/v) and decreasing the amounts of PEG 400. In PK studies, intravenous administration of formulation with particle size <100 nm to mice resulted in a two-fold increase in area under the plasma concentration-time curve (AUClast) and concentration at time zero (C0), there by facilitating the selection of suitable formulation for further efficacy studies.

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