Experimental logP, logD, logS, pKa analysis

 

Assessing the physicochemimal properties of drug compounds are typical early steps in drug discovery. A good compound has high enough water solubility to be able to dissolve to blood/plasma and other aqueous bodily fluids, whilst also having certain amount of lipophilicity to permeate across biological membranes, like the intestine wall (especially important for compounds aimed for oral dosing).

Lipophilicity (logD, logP)

Lipophilicity tells about the compounds ability to dissolve into lipohilic (non-aqueous) solutions. Lipophilicity is needed for the compounds to permeate through the various biological membrane. Lipophilicity is typically measured as the compounds distribution between non-aqueous (octanol) and aqueous (water) phase and the result is expressed as a 10-base logarithm of the concentration ratios between these phases (partition coefficient), logP. A desired logP value (octanol-water partition coefficient) is no more than 5 (also part of the so-called Lipinski rule-of-five; logP 5 = 1:100,000 concentration difference between water and octanol phases).

Another common measure for lipophilicity is the distribution coefficient, logD. lodD takes into account the compounds ionized and non-ionized forms, and therefore the measurement is done at different pH. For un-ionizable compounds, log P = log D at any pH and on the other hand, logP is the octanol-water partition for the neutral (un-ionized) form of the compound. Amongst the different pH values, typically the most interesting is pH 7.4, the physiological pH value.

For analysis of the compound’s division between water and octanol phases, we routinely use UPLC-UV or MS quantitation, depending on the presence of UV absorbing groups in the compound.

logD 1Lipophilicity of the study compounds as a function of pH

Solubility

Solubility is another common physicochemical parameter for drug discovery compounds. Determination of the aqueous solubility of the drug candidate is an important analysis as it reflects the bioavailability of the compound. Especially with oral drugs the poor solubility limits the absorption of compound from the gastrointestinal tract. Solubility is often a challenge for recent drug discovery compounds. Poor solubility is also affecting other AMDE/DMPK analysis, if some fraction of the compound precipitates and is unavailable e.g. in assays for metabolite stability and various CYP identification/inhibition/induction assays.

We measure solubility typically in PBS at various pH values (including physiological Ph 7.4) by first sonicating the solution in a water bath 37oC and left to stand at room temperature for 16 hours followed by filtration and analysis with UPLC-UV or MS, depending on the presence of UV absorbing groups in the compound.

logSSolubility of the study compounds as a function of pH

pKa

The acid dissociation constant, Ka, is a mesure of the strength of the acid and its typically expressed as pKa (-log10(ka). Drug compounds are almost always weak acids or bases and depending on the pH, they are thus either charged (A- or HB+) or neutral. The neutral form has better lipophilicity and thus better permeability whilst the charged form has better aqueous solubility. pKa values gives the pH above which the compound is neutral (bases; B) or charged (acids; A-) and below which the compound is neutral (acids; HA) or charged (bases; HB+).

We analyse the compounds pKa values by calculating it from the logS or logD values, measured in 10 different pH values between pH 2…10 (including pH 6.5 and pH 7.4).

Our services in physicochemical analysis

We provide the expertise in analysing the common physicochemical properties of compounds (logS, logP, logD, pKa). As these are typically done in the very early stages of drug discovery, it is often best to analyze a set of compounds rather than individual compound. We routinely conduct physicochemical analysis on weekly basis. Depending on our customer’s needs, we can do both the full reporting of the study or just analysing the selected properties and reporting them in a spreadsheet format.