Preliminary SAR of Novel Pleuromutilin-Polyamine Conjugates

Abstract

While pleuromutilin (1) and its clinically available derivatives (2-6) are highly effective against Gram-positive bacteria, they remain inactive against many pathogenic Gram-negative bacteria due to the efflux pump AcrAB-TolC. In an effort to broaden the spectrum of activity of pleuromutilin (1), we developed a series of novel pleuromutilin-polyamine conjugates (9a-f) which exhibited promising intrinsic antimicrobial properties, targeting both Gram-positive and Gram-negative bacteria, including Staphylococcus aureus, methicillin-resistant S. aureus (MRSA), and Escherichia coli, along with the fungal strain Cryptococcus neoformans, and were devoid of cytotoxic and hemolytic properties with the exception of one conjugate. Furthermore, this series displayed moderate to low antibiotic potentiation of legacy antibiotics doxycycline and erythromycin, with three conjugates enhancing the activity four-fold in combination with doxycycline. In comparison to pleuromutilin (1) and tiamulin (2), one of the conjugates exhibited an expanded spectrum of activity, including Gram-negative bacteria and fungi, making it a promising option for combating microbial infections.

Keywords: antibiotic enhancement; antimicrobial; membrane disruption; pleuromutilin; polyamine.

Figure 1

Structures of pleuromutilin (1) and its derivatives (2–6).

Scheme 1

Synthesis of pleuromutilin O-22-tosylate (8) and tiamulin (2).

Figure 2

Boc-protected polyamines 7a–f.

Scheme 2

General method for the synthesis of target pleuromutilin–polyamine conjugates (9a–f).

Figure 3

Bacterial growth inhibition exhibited by 2 (left) and 9e (right) against S. aureus ATCC 25923 at different concentrations. Positive control was bacteria only and negative control was media only.

Figure 4

ATP release in S. aureus ATCC 25923 (left) and MRSA (right) exhibited by polyamine conjugate 9e as determined using ATP efflux assay. Squalamine (100 µg/mL) was the positive control and water was the negative control. Compounds were tested at a final concentration of 100 µg/mL, and results are reported as percentage (%) relative to positive control.

Figure 5

The ability of tiamulin (2) (left) and polyamine conjugate 9e (right) to act as membrane disruptors in P. aeruginosa PAO1, as determined using a nitrocefin hydrolysis assay. Polymixin B (PMB) was the positive control (98.3 μM, 128 μg/mL), and the negative control was bacteria with nitrocefin.