Estimation of skin permeation by liquid chromatography

Dermal absorption is a key process in the drug delivery studies of the pharmaceutical and cosmetic industries, as well as in the fields of dermal toxicology, risk assessment, and the exposure of environmental pollutants. This process is typically described by the skin-water permeability coefficient. However, in vivo determination is laborious and expensive. Thus, in the last few years, the development of prediction models from structure descriptors or subrogation through physico-chemical measurements has gained interest. In the present work, a previous subrogation model based on the chromatographic retention on a common C18 column has been tested for a wide set of drugs with very different chemical nature and having a wide range of permeability values. A total of 65 compounds have been used to establish the correlation between skin permeation and the HPLC retention, corrected by the McGowan volume of the drug. Afterwards it was successfully validated in terms of robustness and prediction ability. Finally, the permeability coefficient was estimated for a set of 29 new drugs, and results compared to the ones obtained by other estimation methods, as well as the available in vitro measured values, with very good agreement

Influence of the acid-base ionization of drugs in their retention in reversed-phase liquid chromatography

The effect of the ionization in the RP-HPLC retention of 66 acid-base compounds, most of them drugs of pharmaceutical interest, is studied. The retention time of the compounds can be related to the pH measured in the mobile phase (pwsH) through the sigmoidal equations derived from distribution of the neutral and ionic forms of the drug into the stationary and mobile phases. Fitting of the obtained retention vs. pH profiles provides the retention times of the ionic and neutral forms and the pKa values of the drugs in the mobile phase (pwsKa). The obtained pwsKa values are linearly correlated to the pKa values in water (pwwKa) with two different correlations, one for neutral acids and another for neutral bases that reflect the different influence of the dielectric constant of the medium in ionization of acids and bases. The retention of the neutral species is well correlated to the octanol-water partition coefficient of the drugs as measure of the lipophilicity of the drug, which affects chromatographic retention. Also, the retention time of the ionized forms is related to the retention time of the neutral forms by two different linear correlations, one for anions and the other for cations. These last correlations point out the different retention behaviour of anions and cations: anions are less retained than cations of the same lipophilicity, as measured by the octanol-water partition coefficient of the neutral form. The different retention behaviour of anionic, cationic and neutral forms is confirmed by the hold-up times obtained from different approaches: pycnometry and retention times of anionic (KBr and KI) and neutral (DMSO) markers. Hold-up times obtained by pycnometric measurements agree with those obtained by retention of neutral markers (0.83-0.85 min), whereas hold-up time for anions is mobile phase pH dependent. At acidic pH it is similar to the hold-up time for neutral markers (0.83 min), but then it decreases with the increase of mobile phase pH to 0.65 min at pH 11. The decrease can be explained by the ionization of the silanols of the column and exclusion of anions by charge repulsion. Although not directly measured, the obtained retention data and correlations indicate hold-up time for cations are similar or slightly lower than hold-up time for neutral compounds (0.77-0.83 min). The model proposed and the correlations obtained can be very useful for its implementation in retention prediction algorithms for optimization of separation purpose

Optimization of experimental conditions for skin-PAMPA measurements

In recent years, the parallel artificial membrane permeability assay (PAMPA) has been extended for prediction of skin permeation by developing an artificial membrane which mimics the stratum corneum structure, skin-PAMPA. In the present work, the different parameters affecting skin-PAMPA permeability, such as incubation time and stirring, have been studied to establish ideal assay conditions to generate quality data for a screening of active pharmaceutical ingredients (API) in early stage drug discovery. Another important parameter is membrane retention, which shows dependence on lipophilicity when compounds are in their neutral form. Furthermore, the stability of the membrane has been investigated at different pH values, especially at basic pHs. Finally, a good correlation between human skin permeability and skin-PAMPA permeability, with a large dataset (n = 46), has been established. The optimized assay conditions were an incubation time of 4 hours with stirring in a pH below 8. With all these considerations the thickness of the aqueous boundary layer is decreased as much as possible and the membrane stability is guaranteed

Critical comparison of shake-flask, potentiometric and chromatographic methods for lipophilicity evaluation (log Po/w) of neutral, acidic, basic, amphoteric, and zwitterionic drugs

In the present study three different procedures have been compared for the determination of the lipophilicity of the unionized species (log Po/w) of neutral, acidic, basic, amphoteric, and zwitterionic drugs. Shake-flask, potentiometric and chromatographic approaches have been assayed in a set of 66 representative compounds in different phases of advanced development. An excellent equivalence has been found between log Po/w values obtained by shake-flask and potentiometry, while the chromatographic approach is less accurate but very convenient for screening purposes when a high-throughput is required. In the case of zwitterionic and amphoteric compounds, either for shake-flask and chromatographic methods, the pH has to be accurately selected in order to ensure the compound to be in its neutral form

Evaluation of logPo/w values of drugs from some molecular structure calculation software

The iridium‐catalyzed asymmetric hydrogenation of several N‐sulfonyl allyl amines is reported. All substrates can be easily obtained by the Ir‐catalyzed isomerization of N‐tosylaziridines reported previously. The commercially available threonine‐derived phosphinite (UbaPHOX) iridium complex has been found to be the best catalyst for this catalytic application, affording β‐methyl amines with good to excellent ee values (up to 94%). The synthetic potential of this novel methodology was demonstrated by the formal synthesis of Lorcaserin and LY‐404187

Linear free energy relationship models for the retention of partially ionized acid-base compounds in reversed-phase liquid chromatography

The LFER model of Abraham is applied to the retention of the neutral and ionic forms of 94 solutes in a C18 column and 40% v/v acetonitrile/water mobile phase. The results show that polarizability and cav- ity formation interactions increase retention, whereas dipole and hydrogen bonding interactions favours partition to the mobile phase and thus, they decrease retention. The coefficients of the ionic descriptors measure the effect of the electrostatic interactions and their contribution to partition of the cation or anion between the two mobile and stationary chromatographic phases. A new LFER model for application to the retention of partially dissociated acids and bases is derived aver- aging the descriptors of the neutral and ionic forms according to their degrees of ionization in the mobile phase. This new LFER model is satisfactorily compared to other literature modified Abraham models for a set of 498 retention data of partially dissociated acids and bases. All tested models require the calculation of the ionization degrees of the compounds at the measuring pH. Calculation of the ionization degrees in the chromatographic mobile phase (i.e. from pH and p K a in the eluent) give good correlations for all tested models. However, estimation of these ionization degrees from pH -p K a data in pure water gives biased estimations of the retention of the partially ionized solutes

Evaluation of logPo/w values of drugs from some molecular structure calculation software

The iridium‐catalyzed asymmetric hydrogenation of several N‐sulfonyl allyl amines is reported. All substrates can be easily obtained by the Ir‐catalyzed isomerization of N‐tosylaziridines reported previously. The commercially available threonine‐derived phosphinite (UbaPHOX) iridium complex has been found to be the best catalyst for this catalytic application, affording β‐methyl amines with good to excellent ee values (up to 94%). The synthetic potential of this novel methodology was demonstrated by the formal synthesis of Lorcaserin and LY‐404187

Critical comparison of shake-flask, potentiometric and chromatographic methods for lipophilicity evaluation (log Po/w) of neutral, acidic, basic, amphoteric, and zwitterionic drugs

In the present study three different procedures have been compared for the determination of the lipophilicity of the unionized species (log Po/w) of neutral, acidic, basic, amphoteric, and zwitterionic drugs. Shake-flask, potentiometric and chromatographic approaches have been assayed in a set of 66 representative compounds in different phases of advanced development. An excellent equivalence has been found between log Po/w values obtained by shake-flask and potentiometry, while the chromatographic approach is less accurate but very convenient for screening purposes when a high-throughput is required. In the case of zwitterionic and amphoteric compounds, either for shake-flask and chromatographic methods, the pH has to be accurately selected in order to ensure the compound to be in its neutral form

Synthesis and Structure–Activity Relationship Study of a New Series of Selective σ₁ Receptor Ligands for the Treatment of Pain: 4‑Aminotriazoles

The synthesis and pharmacological activity of a new series of 4-aminotriazoles as potent σ₁ receptor (σ₁R) ligands are reported. The compounds were prepared using a 4–5-step process, involving as a key step a click chemistry reaction between ynamides and azides. The most active compounds exhibited nanomolar potency for the σ₁R, and the selectivity over the σ₂R was improved on decreasing the central amine basicity. It was concluded that in order to achieve good σ₁R potency a minimum lipophilicity was required, while limiting to a defined range of cLog<i>P</i> avoided human ether-a-go-go-related gene channel inhibition. This made the most interesting derivatives to be concentrated in a narrow margin of lipophilicity. Among them, compound <b>13g</b> exhibited the most potent in vivo antinociceptive properties, which are indicative of its antagonist character

Synthesis and Biological Evaluation of the 1‑Arylpyrazole Class of σ₁ Receptor Antagonists: Identification of 4‑{2-[5-Methyl-1-(naphthalen-2-yl)‑1<i>H</i>‑pyrazol-3-yloxy]ethyl}morpholine (S1RA, E‑52862)

The synthesis and pharmacological activity of a new series of 1-arylpyrazoles as potent σ₁ receptor (σ₁R) antagonists are reported. The new compounds were evaluated in vitro in human σ₁R and guinea pig σ₂ receptor (σ₂R) binding assays. The nature of the pyrazole substituents was crucial for activity, and a basic amine was shown to be necessary, in accordance with known receptor pharmacophores. A wide variety of amines and spacer lengths between the amino and pyrazole groups were tolerated, but only the ethylenoxy spacer and small cyclic amines provided compounds with sufficient selectivity for σ₁R vs σ₂R. The most selective compounds were further profiled, and compound <b>28</b>, 4-{2-[5-methyl-1-(naphthalen-2-yl)-1<i>H</i>-pyrazol-3-yloxy]­ethyl}­morpholine (S1RA, E-52862), which showed high activity in the mouse capsaicin model of neurogenic pain, emerged as the most interesting candidate. In addition, compound <b>28</b> exerted dose-dependent antinociceptive effects in several neuropathic pain models. This, together with its good physicochemical, safety, and ADME properties, led compound <b>28</b> to be selected as clinical candidate