Salt Screening - Motivation and Theory - Solid Form Screening
Motivation
Sponsors may require a salt screen for a number of reasons, or a combination of these:
• Solubility
In general the aqueous solubility of a salt will be greater than the API because of the polar nature of charged species, salts retain their charge in aqueous solution. The current trend is for APIs to become more lipophilic and it is common to run into aqueous solubility issues with compounds that have been designed for potency by optimisation of lipophilic interactions. Exceptions to high aqueous solubility are salts made with high MW, lipophilic (and hence highly aqueous insoluble) acids such as naphthalene sulfonic acids. Such examples will dominate the properties of the salt produced. They may be useful, however, to generate crystallinity in otherwise reluctant examples.
• Stability
Common causes of drug substance instability are oxidation, hydrolysis and inter- or intra- molecular reactions of the API itself. Salt formation can have an influence on stability by reducing the nucleophilicity of basic centres to shut down the majority of these processes. Hygroscopicity, i.e. form instability to high humidity will vary amongst different salts.
• Crystallinity
It is reasonable to assume that salts will have greater crystallinity and higher melting point than the parent material. Sponsor may also be seeking the first crystalline forms of a compound that is amorphous as the free API. Crystalline forms are generally more stable, easier to handle and show greater reliability in formulation and pharmaceutical performance.
• Novelty
A novel salt form is patentable if it has some demonstrable advantage over the prior art and the inventor has a proven method of preparation, i.e. reduction to practice. It is therefore prudent to look for optimal solid forms that offer little scope for attack from competitors who may see an opportunity to exploit a flawed profile.
• Other processability
This can include issues in large scale synthesis such as slow filtration, and further downstream processes such as compressibility etc.
Theory
By definition a salt is formed when proton transfer takes place from an acid to a base. About 50% of all marketed drugs are sold as salt forms. Acidic drugs present fewer options for salt formation, many basic compounds that could in principle be used to make salts of acids are biologically active (some highly toxic). Lithium is used as a drug to treat manic depression and many other metals are toxic.
Most common salts used in Pharma materials
Salt formation and pKa
The majority of requests for salt screening will involve basic compounds. Basic groups are used in drug design either to increase potency by binding to an acidic centre on the receptor or with the deliberate aim of accessing salt formation to improve aqueous solubility. The popularity of this latter approach has lessened in recent times with the discovery in clinical trials of a rare but serious cardiotoxicity associated with basic, lipophilic compounds (Torsades de Pointes, Herg activity).
It is often stated that salt formation is viable if there is a >2 pKa unit difference between the base and the acid, i.e. proton transfer is energetically favourable. Note however that the pKa values used in such estimations are measured or simulated in water and will not necessarily hold under the nonaqueous conditions used for screening. This effect is particularly marked for carboxylic acids which are 5 pKa units (or more) less acidic in solvents other than water. See the cocrystal paper for further discussion on the consequences of this effect.
Selection of acids for salt screening
Selection of acids uses 5 criteria:
• pKa
The strength of any acid depends on the stabilisation of the respective anion by electronic effects and by solvation. The acids routinely used in salt screening range from strong (low pKa) acids (hydrochloric, sulphuric, phosphoric, sulfonic) to weak acids (acetic, propionic) with a range in between of substituted aliphatic and aromatic carboxylic acids.
• Functional Group
Selection of acids for salt screening should include as wide a range of functional groups as possible, e.g HCl, sulfonic acids, phosphonic acids, carboxylic acids and this principle also applies to the respective anion which should also cover as wide a range of structural motifs as possible. For example sulfonic acids should include sulphuric acid, methane sulfonic acid, toluene sulfonic acid etc. Carboxylic acids are available in considerable diversity including simple and substituted alkyl, diand tri- acids, and aromatic systems. This makes carboxylic acids popular choices for salt screening but care should be taken in selection of these as they will vary in pKa. Salts of weaker bases claimed with carboxylic acids may well be better described and evaluated as cocrystals and this is not a trivial distinction (see the cocrystal paper). The most acidic carboxylic acids, and hence those most likely to make salts include oxalic, maleic, and citric acid. Polybasic acids such as these should also be included in screens in varying stoichiometry.
• Chemical compatibility
It is particularly important to assess the reactivity of the API prior to selecting acids for salt screening. One aspect to consider is the presence of any latent electrophilic centre in the structure which may react with a nucleophilic anion generated from the acid, particularly after protonation of the API. Examples are Michael acceptors, esters, and heterocylic ethers. Bromide is a stronger nucleophile than chloride and this is one of the reasons hydrochlorides are much more common than hydrobromide salts.
Experienced synthetic and medicinal chemists will be able to assess a structure for liability to react with acids and therefore evaluate the risks associated with the presence of excess acid in the salt formation. Electron-rich aromatic and benzylic functions are common acid sensitive functions.
One common combination that should be avoided on grounds of compatibility is sulfonic acid/ low MW alcohol. This can generate alkyl sulfonates that are powerful alkylating agents and can either react with the API or themselves be high risk impurities due to their carcinogenicity. If this combination is unavoidable isopropanol is a much safer option than methanol, but it is still prudent to avoid long reaction times/heating.
• Solubility
Free acid solubility is not a very reliable basis on which to choose possible acids for salt formation. Formation of the salt requires proton transfer and the pair of ions thus formed will have very different solubility to the neutral parents. Typically the salt will have higher solubility in polar solvents and lower solubility in lipophilic systems. This can be useful in driving crystallisation by addition of antisolvents (heptane, TBME, toluene etc)
There is an exception to this generalisation when the acid is highly lipophilic and insoluble (e.g. naphthalene sulfonic acid) where the properties of the acid will dominate the salt, this can be useful in cases where the API is extremely soluble in the usual solvents.
• Safety
Early in development the salt selections will be based more on solubility and stability than safety, the concept of ‘fit-for-purpose’ is useful here. Although it is tempting to think that a carefully considered salt form can be identified early and carried throughout development, in reality priorities change and an acceptable profile for rat studies may not be sufficiently benign for clinical use. This needs to be balanced with the cost of changing form late in development where further toxicity and equivalence studies will be necessary. There is value, therefore in maintaining studies on solid form in parallel with development progress to ensure the widest range of options are available to address any formulation or toxicity issues.
Salts and polymorphism
As with all crystalline materials, salts may exist as different polymorphs. It has been stated (McCrone) that the number of polymorphs discovered is proportional to the time spent looking for them. It is widely believed that salts are more prone to polymorphism than cocrystals, perhaps due to the strength of the salt interaction being able to support a greater variety of structures but there is little evidence to suggest this is generally true.
‘Permanent’ charges
There are 2 classes of compound that present a different challenge for salt form screening:
• Highly basic compounds such as guanidines, these are nature’s strong base with a pKa around 12.5 so will usually be present in the protonated form.
• Quaternary salts such as tetraalkylammonium or phosphonium species.
Salt screening for these compounds will comprise an ‘anion swap’ using ion exchange resins and requires an experienced specialist approach.
Salt characterisation
The following methods are useful in confirming crystalline salt formation and stoichiometry:
• PXRD to confirm crystallinity.
• DSC for melting point and stability.
• NMR (1H) for evidence of protonation and presence of acid (where possible e.g. mesylate, citrate, fumarate). Also detects solvation.
• CHN elemental analysis, useful for larger anions not detectable by 1H NMR (phosphate, sulphate).
• Ion Chromatography is available in LGC to directly measure inorganic anions.
In addition the sponsor may be interested in further properties:
• TGA for degree of solvation/hydration and stability.
• DVS Hygroscopicity/hydration is an important consideration for salts which can have high affinity for water. Typically high on sponsors list of interests.
• Dissolution/solubility. Preliminary assessment compared to API is often requested in water or biorelevant buffers. Not usually included in screening.
• Stability. Particularly relevant where a possible lability to acid is suspected. A simple stability study at a fixed temperature monitored by HPLC is sufficient.
Download the Salt Screening and Selection Theory Sheet PDF
Salt Screening - Motivation and Theory PDF
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