The objective of this study was to investigate the risk of attenuated efficacy due to adaptive resistance for the siderophore-conjugated monocarbam SMC-3176 in Pseudomonas aeruginosa by using a pharmacokinetic/pharmacodynamic (PK/PD) approach. MICs were determined in cation-adjusted Mueller-Hinton broth (MHB) and in Chelex-treated, dialyzed MHB (CDMHB). Spontaneous resistance was assessed at 2؋ to 16؋ the MIC and the resulting mutants sequenced. Efficacy was evaluated in a neutropenic mouse thigh model at 3.13 to 400 mg/kg of body weight every 3 h for 24 h and analyzed for association with free time above the MIC (fT>MIC). To closer emulate the conditions of the in vivo model, we developed a novel assay testing activity mouse whole blood (WB). All mutations were found in genes related to iron uptake: piuA, piuC, pirR, fecI, and pvdS. Against four P. aeruginosa isolates, SMC-3176 displayed predictable efficacy corresponding to the fT>MIC using the MIC in CDMHB (R 2 ‫؍‬ 0.968 to 0.985), with stasis to 2-log kill achieved at 59.4 to 81.1%. Efficacy did not translate for P. aeruginosa isolate JJ 4-36, as the in vivo responses were inconsistent with fT>MIC exposures and implied a threshold concentration that was greater than the MIC. The results of the mouse WB assay indicated that efficacy was not predictable using the MIC for JJ 4-36 and four additional isolates, against which in vivo failures of another siderophore-conjugated ␤-lactam were previously reported. SMC-3176 carries a risk of attenuated efficacy in P. aeruginosa due to rapid adaptive resistance preventing entry via the siderophore-mediated iron uptake systems. Substantial in vivo testing is warranted for compounds using the siderophore approach to thoroughly screen for this in vitro-in vivo disconnect in P. aeruginosa.

The type 4 cAMP-specific phosphodiesterases (PDE4s) are Mg(2+)-dependent hydrolases that catalyze the hydrolysis of 3', 5'-cAMP to AMP. Previous studies indicate that PDE4 exists in two conformations that bind the inhibitor rolipram with affinities differing by more than 100-fold. Here we report that these two conformations are the consequence of PDE4 binding to its metal cofactor such as Mg(2+). Using a fluorescence resonance energy transfer (FRET)-based equilibrium binding assay, we identified that L-791,760, a fluorescent inhibitor, binds to the apoenzyme (free enzyme) and the holoenzyme (enzyme bound to Mg(2+)) with comparable affinities (K(d) approximately 30 nM). By measuring the displacement of the bound L-791,760, we have also identified that other inhibitors bind differentially with the apoenzyme and the holoenzyme depending upon their structure. CDP-840, SB-207499, and RP-73401 bind preferentially to the holoenzyme. The conformational-sensitive inhibitor (R)-rolipram binds to the holoenzyme and apoenzyme with affinities (K(d)) of 5 and 300 nM, respectively. In contrast to its high affinity (K(d) approximately 2 microM) and active holoenzyme complex, cAMP binds to the apoenzyme nonproductively with a reduced affinity (K(d) approximately 170 microM). These results demonstrate that cofactor binding to PDE4 is responsible for eliciting its high-affinity interaction with cAMP and the activation of catalysis.

Type 4 phosphodiesterases (PDE4s) are metallohydrolases that catalyze the hydrolysis of cAMP to AMP. At the bottom of its active site lie two divalent metal ions in a binuclear motif which are involved in both cAMP binding and catalysis [(2000) Science 288, 1822-1825; (2000) Biochemistry 39, 6449-6458]. Using a SPA-based equilibrium [(3)H]rolipram binding assay, we have determined that Mg(2+), Mn(2+), and Co(2+) all mediated a high-affinity (K(d) between 3 and 8 nM) and near stoichiometric (R)-rolipram binding to PDE4. In their absence, (R)-rolipram binds stoichiometrically to the metal ion-free apoenzyme with a K(d) of approximately 150 nM. The divalent cation dose responses in mediating the high-affinity rolipram/PDE4 interaction mirror their efficacy in catalysis, suggesting that both metal ions of the holoenzyme are involved in mediating the high-affinity (R)-rolipram/PDE4 interaction. The specific rolipram binding to the apo- and holoenzyme is differentially displaced by cAMP, AMP, and other inhibitors, providing a robust tool to dissect the components of metal ion-dependent and independent PDE4/ligand interactions. cAMP binds to the holoenzyme with a K(s) of 1.9 microM and nonproductively to the apoenzyme with a K(d) of 179 microM. In comparison, AMP binds to the holo- and apoenzyme with K(d) values of 7 and 11 mM, respectively. The diminished Mg(2+)-dependent component of AMP binding to PDE4 suggests that most of the Mg(2+)/phosphate interaction in the cAMP/PDE4 complex is disrupted upon the hydrolysis of the cyclic phosphoester bond, leading to the rapid release of AMP.

The PDE4 catalytic machinery comprises, in part, two divalent cations in a binuclear motif. Here we report that PDE4A4 expressed in Sf9 cells exhibits a biphasic Mg 2+ doser esponse (EC 50 of V V0.15 and s 10 mM) in catalyzing cAMP hydrolysis. In vitro phosphorylation of PDE4A4 by the PKAcatalytic subunit increases the enzyme's sensitivity to Mg 2+ , leading to 4-fold increased cAMP hydrolysis without affecting its K m . The phosphorylation also increases the potencies of (R)-and (S)-rolipram without affecting CDP-840 and SB-207499. The results support that modulating the cofactor binding affinity of PDE4 represents a mechanism for regulating its activity. ß 2002 Published by Elsevier Science B.V. on behalf of the Federation of European Biochemical Societies.

No abstract

No abstract

No abstract

No abstract