Relevant PK Research Articles

Structural and pH Dependence of Excited State PCET Reactions Involving Reductive Quenching of the MLCT Excited State of [RuII(bpy)2(bpz)]2+ by Hydroquinones

The proton-coupled electron transfer (PCET) reaction between the bpz-based photoexcited (3)MLCT state of [Ru(II)(bpy)(2)(bpz)](2+) (bpy = 2,2'-bipyridine, bpz = 2,2'-bipyrazine) and a series of substituted hydroquinones (H(2)Q) has been studied by transient absorption (TA) and time-resolved electron paramagnetic resonance (TREPR) spectroscopy at X-band. When the reaction is carried out in a CH(3)CN/H(2)O mixed solvent system with unsubstituted hydroquinone, the neutral semiquinone radical (4a) and its conjugate base, the semiquinone radical anion (4b), are both observed. Variation of the acid strength in the solvent mixture allows the acid/base dependence of the PCET reaction to be investigated. In solutions with very low acid concentrations, TREPR spectra exclusively derived from radical anion 4b are observed, while at very high acid concentrations, the spectrum is assigned to the protonated structure 4a. At intermediate acid concentrations, either a superposition of spectra is observed (slow exchange between 4a and 4b) or substantial broadening in the TREPR spectrum is observed (fast exchange between 4a and 4b). Variation of substituents on the H(2)Q ring substantially alter this acid/base dependence and provide a means to investigate electronic effects on both the ET and PT components of the PCET process. The TA results suggest a change in mechanism from PCET to direct ET quenching in strongly basic solutions and with electron withdrawing groups on the H(2)Q ring system. Changing the ligand on the Ru complex also alters the acid/base dependence of the TREPR spectra through a series of complex equilibria between protonated and deprotonated hydroquinone radicals and anions. The relative intensities of the signals from radical 4a versus 4b can be rationalized quantitatively in terms of these equilibria and the relevant pK(a) values. An observed equilibrium deuterium isotope effect supports the conclusion that the post-PCET HQ(•)/Q(•-) equilibrium is the most important in determining the 4a/4b ratio at early delay times.

Ionization Behavior of Amino Lipids for siRNA Delivery: Determination of Ionization Constants, SAR, and the Impact of Lipid pKa on Cationic Lipid−Biomembrane Interactions

Ionizable amino lipids are being pursued as an important class of materials for delivering small interfering RNA (siRNA) therapeutics, and research is being conducted to elucidate the structure-activity relationships (SAR) of these lipids. The pK(a) of cationic lipid headgroups is one of the critical physiochemical properties of interest due to the strong impact of lipid ionization on the assembly and performance of these lipids. This research focused on developing approaches that permit the rapid determination of the relevant pK(a) of the ionizable amino lipids. Two distinct approaches were investigated: (1) potentiometric titration of amino lipids dissolved in neutral surfactant micelles; and (2) pH-dependent partitioning of a fluorescent dye to cationic liposomes formulated from amino lipids. Using the approaches developed here, the pK(a) values of cationic lipids with distinct headgroups were measured and found to be significantly lower than calculated values. It was also found that lipid-lipid interaction has a strong impact on the pK(a) values of lipids. Lysis of model biomembranes by cationic lipids was used to evaluate the impact of lipid pK(a) on the interaction between cationic lipids and cell membranes. It was found that cationic lipid-biomembrane interaction depends strongly on lipid pK(a) and solution pH, and this interaction is much stronger when amino lipids are highly charged. The presence of an optimal pK(a) range of ionizable amino lipids for siRNA delivery was suggested based on these results. The pK(a) methods reported here can be used to support the SAR screen of cationic lipids for siRNA delivery, and the information revealed through studying the impact of pK(a) on the interaction between cationic lipids and cell membranes will contribute significantly to the design of more efficient siRNA delivery vehicles.

Phosphate ester cleavage promoted by a tetrameric iron(III) complex

The purple acid phosphatases (PAPs) are the only binuclear metallohydrolases where the necessity for a heterovalent active site [Fe(III)-M(II) (MisFe, Zn or Mn)] for catalysis has been established. The paradigm for the construction of PAP biomimetics, both structural and functional, is that the ligands possess characteristics which mimic those of the donor sites of the metalloenzyme and permit discrimination between trivalent and divalent metal ions. The donor atom set of the ligand 2-((2-hydroxy-5-methyl-3-((pyridin-2-ylmethylamino)methyl)benzyl)(2-hydroxybenzyl)amino)acetic acid (H(3)HPBA) mimics that of the active site of PAP although the iron(III) complex of this ligand has been characterized as the tetramer [Fe(4)(HPBA)(2)(μ-CH(3)COO)(2)(μ-O)(μ-OH)(OH(2))(2)]ClO(4)·5H(2)O. The phosphoesterase-like activity of the complex in 1:1 acetonitrile/water has now been investigated using the substrate 2,4-bis(dinitrophenyl)phosphate. The pH dependence of the catalytic rate revealed a non-symmetric bell-shaped profile, with a finite but non-zero rate at high pH. Unlike the traditional approach usually employed to analyse these bell-shaped profiles, the approach used here involved incorporating additional species which contribute to the overall activity. Employing this approach, we show that the complex has a k (cat) of 1.6 (±0.2)×10(-3)s(-1), three kinetically relevant pK (a) values of 5.3, 6.2 and 8.4, with K (M) of 7.4±0.6mM. The kinetic parameters are similar to those reported for heterovalent PAP biomimetics. Additionally, it is observed that, unlike the enzyme, the oxidation state is not the determining factor for catalytic activity.

Parathyroid Hormone (1-34)-Coated Microneedle Patch System: Clinical Pharmacokinetics and Pharmacodynamics for Treatment of Osteoporosis

To evaluate the clinical PK/PD of PTH(1-34) delivered by a novel transdermal drug-coated microneedle patch system (ZP-PTH) for the treatment of osteoporosis. Phase 1 PK studies evaluated the effect of site of administration, patch wear time and dose in normal volunteers, ages 40-85yrs. Phase 2 was conducted in post-menopausal women with osteoporosis to determine the patch dose response compared to placebo patch and FORTEO® injection. Phase 1 ZP-PTH patch delivery demonstrated a rapid PTH plasma pulse profile with T(max) 3 times shorter and apparent T(1/2) 2 times shorter than FORTEO®. In Phase 2, ZP-PTH 20, 30 and 40µg doses showed a proportional increase in plasma PTH AUC. Inter-subject and intra-subject AUC variability was similar for all patch doses and comparable to injection. All patch doses produced a significant increase in spine bone mineral density. Unexpectedly, ZP-PTH also produced an early increase in hip bone mineral density, an effect not observed with the injection. These studies suggest that this novel ZP-PTH patch system can deliver a consistent and therapeutically relevant PTH PK profile. Based on encouraging Phase 2 safety and efficacy data, the program is advancing into a pivotal Phase 3 clinical study.

Structural and Catalytic Characterization of a Heterovalent Mn(II)Mn(III) Complex That Mimics Purple Acid Phosphatases

The binuclear heterovalent manganese model complex [Mn(II)Mn(III)(L1)(OAc)(2)] ClO(4) x H(2)O (H(2)L1 = 2-(((3-((bis(pyridin-2-ylmethyl)amino)methyl)-2-hydroxy-5-methylbenzyl)(pyridin-2-ylmethyl)amino)-methyl)phenol) has been prepared and studied structurally, spectroscopically, and computationally. The magnetic and electronic properties of the complex have been related to its structure. The complex is weakly antiferromagnetically coupled (J approximately -5 cm(-1), H = -2J S(1) x S(2)) and the electron paramagnetic resonance (EPR) and magnetic circular dichroism (MCD) spectra identify the Jahn-Teller distortion of the Mn(III) center as predominantly a tetragonal compression, with a significant rhombic component. Electronic structure calculations using density functional theory have confirmed the conclusions derived from the experimental investigations. In contrast to isostructural M(II)Fe(III) complexes (M = Fe, Mn, Zn, Ni), the Mn(II)Mn(III) system is bifunctional possessing both catalase and hydrolase activities, and only one catalytically relevant pK(a) (= 8.2) is detected. Mechanistic implications are discussed.

Expanding the pH Range of Metal−Nucleobase Complexes for Acid−Base Chemistry: Properties of Bis(guanine) Complexes of (bpy)PtIIwith Either Two Major or Major and Minor Tautomers Bonded Simultaneously

Metal nucleobase complexes provide a potential for acid-base chemistry in the physiological pH range and consequently may contribute or be actively involved in catalytic reactions of nucleic acids, notably of RNAs. Expansion of the available pH range is achieved if additional ligands are involved, for example, an aqua ligand or a second nucleobase, and if relevant pK(a) values are sufficiently close. Two bpy's (bpy = 2,2'-bipyridine) containing Pt(II) complexes have been studied in this context: [Pt(bpy)(9-MeGH-N7)(2)](NO(3))(2) x H(2)O (2) and Pt(bpy)(9-EtG-N7)(9-EtG-N1) x 3 H(2)O (3') (with 9-MeGH = 9-methylguanine; 9-EtGH = 9-ethylguanine). Relevant pK(a) values, as determined by pD-dependent (1)H NMR spectroscopy in D(2)O, of the neutral guanine ligands were found to be ca. 7.78 +/- 0.01 and 8.38 +/- 0.01 for 2, yet 4.00 +/- 0.03 and 7.7 +/- 0.1 for 3' (values converted to H(2)O) for each of the two guanine ligands. These values suggest that complex 3' provides a pH range of roughly 4-8 for potential acid-base chemistry, and furthermore that in favorable cases compounds with two ionizable ligands can function as an acid and a base simultaneously. X-ray crystal structures of both 2 and 3' are presented and, in addition, that of [Pt(bpy)(9-EtGH-N7)(2)](NO(3))(2) x (9-EtGH) x 5 H(2)O (2''). Regarding the use of (1)H NMR spectroscopy for the determination of pK(a) values, we note that chemical shifts referenced to sodium 3-(trimethylsilyl)propanesulfonate must be treated with caution when applying cationic complexes because of the possibility of ion pairing. It can lead to mistakes in chemical shift values.

Cadmium(II) complexes of the glycerophosphodiester-degrading enzyme GpdQ and a biomimetic N,O ligand

The glycerophosphodiester-degrading enzyme GpdQ from Enterobacter aerogenes is a promising bioremediator owing to its ability to degrade some organophosphate pesticides and diester products originating from the hydrolysis of nerve agents such as VX. Here, the cadmium derivative of GpdQ was prepared by reconstituting the apoenzyme. Catalytic measurements with (Cd(2+))(2)-GpdQ and the phosphodiester substrate bis(4-nitrophenyl)phosphate yield k(cat) = 15 s(-1). The pK(a) of 9.4, determined from the pH dependence of the catalytic activity, implicates a hydroxide ligand as the catalytic nucleophile. Also prepared was the cadmium-containing biomimetic [Cd(2)((HP)(2)B)(OAc)(2)(OH(2))](PF(6)) (where (HP)(2)B is [2,6-bis([(2-pyridylmethyl)(2-hydroxyethyl)amino]methyl)-4-methylphenol]), which mimics the asymmetry of the metal ion coordination in the active site of GpdQ. The phosphoesterase-like activity of [Cd(2)((HP)(2)B)(OAc)(2)(OH(2))](PF(6)) was studied using the substrate bis(2,4-dinitrophenyl)phosphate, yielding a kinetically relevant pK(a) of 8.9, with k(cat) = 0.004 s(-1). In summary, the model is both an adequate structural and a reasonable functional mimic of GpdQ.