Efficiently Solubilise New Drug Products Biology Essay

Published: 23, March 2015

It is important to be able to quickly and efficiently solubilise new drug products, as apart from the delivery and bioavailability benefits, undesirable properties may be masked and only come to light after avoidable time and expenditure, poor lipophilicity, permeability and pre-systematic metabolism[4] may all be more difficult to detect.The increasingly common poor solubility of drug leads has led to a new focus on development strategies.

More than 40% drugs screened through combinational and high throughput screening (HTS) processes are poorly soluble in water.[1][2] In a trend illustrated by Opreas[2] review of the area, he demonstrated that >50% of possible lead candidates in the sample set, leads being identified by possessing an activity of >1 nanomolar, are high molecular weight >425g/mol, poorly soluble, LogSw <-4.25 and posses a LogP >4.25. This presents challenges in achieving a satisfactory bioavailability and dosage for the product to be marketed.

Merck and Pfizer are both undergoing a trend of developing increasingly large lead drug candidates.[3] With the likely accompaniment solubility issues associated with larger molecules.

In the period of 2007-2010 a fifth of new molecular entities (NME) approved by the FDA were listed as practically insoluble (this number would likely rise as a number of drugs approved were delivered as organic emulsions or applied dermally for which the solubility was not stated) and 44% had a molecular weight over 500. (fig 1)

Figure 1. Molecular weights and numbers of the 79 NME's approved by the FDA in the period of 2007-2010

For the above reasons numerous delivery and solubilising strategies have been developed; salt formation, prodrug strategies, cosolvents, micronisation, cyclodextrins and liposomes have all proven successful. There is also newer nanosizing, polymeric micelles, asymmetric membrane capsules and more refined crystal engineering processes to consider as possible strategies, not to mention that many of these techniques are combinable leading to more permutations.

There is constant research in this area and as such there is a constant increasing sophistication and number of techniques to consider. This review will consider a selection of these techniques and hopefully prove useful to the reader in selecting a route to proceed when presented with the difficulties of poorly soluble drugs.

The methodology applied to this problem can broadly be separated into two categories, formulation and chemical. Chemical methods modify the drug molecule whereas formulation methods use non-covelent approaches to modify the delivery environment.

2. Formulation Approaches

2.1. Micronisation

Micronisation, usually through jet milling or cogrindinging, is a proven technology for improving dissolution rate of drugs through increased surface area.[11][12][13][14] This is explained by the Brummer and Levich modification of the Noyes Whitney equation[15][16] demonstrating that surface area of a drug is directly proportional to its rate of dissolution-

(1)

X is the amount of drug in solution, t is time A is the effective surface area, D is the diffusion coefficient, δ is the effective boundary layer, Cs is the saturation solubility of the drug and V is the volume of the dissolution medium.

In the Biopharmaceutics Classification System drugs are separated into four groups, this review will only consider type II 'grease ball'[5], high permeability, low solubility, and type IV 'brick dust', low permeability, low solubility drugs.

Type II high lipophilicity drugs often perform better than expected from their water solubility due to the ease of which they pass through biomembranes and the increased dissolution rate in the presence of bile salts in the intestinal tract[6][7] For these drugs the solvation rate is the rate limiting step and often simply improving the dissolution rate of the drug is sufficient to increase its bioavailability.[7][8][11][12][13][14]

Via this mechanism it can also reduce the variable performance and bioavailability issues in oral administration between the fed/fasted states of patients.[6][7] Unfortunately this offers little in itself in the way of improving bioavailability of type IV drugs, or drugs which have an extremely low solubility. Drugs for which the rate of adsorption is not fast enough for effective administration.

2.2. Nanosizing

Nanosizing is the logical next step in the process. One approach is to use nanocrystals, usually in a colloidal suspension or formulated to redisperse into one. That is drug nano-crystals composed of 100% drug-substance, no matrix material and the mean particle size being below a micron. These often also include a surface stabiliser, the reason and choice of stabiliser will be discussed later. They can often be further processed to give a solid dosage form.[1][17][18][19]

The main reason for micronisation of API's is the increased bioavailability due to increased dissolution rate, the same is still true for nanoparticles, with a more pronounced effect, though due to electrostatics and Vann der Waals interactions the effective surface area of particles is effectively capped. There are however several other properties unique to nanoparticles which increase their attractiveness as a formulation solution.

2.2.1. Size and Solubility

The somewhat unintuitive fact that Cs, the saturation solubility, increases with smaller particle size[1][16][17][18][20][22][23][25][34][36][37][38][39] is a major advantage over other techniques as it relies on a physical property of the drug and not a chemical property. Therefore nanosizing could be applied to almost any solid, making it nearly always applicable. This means that it could be applied unilaterally across all new possible leads and drug substances, for example to quickly form a nanosuspension, which need only be stable for a few days for parenteral activity testing in drug discovery. A high pressure homogenisation technique could be used for the production of the suspension; the solvent can be organic or aqueous making this flexible. A controlled precipitation process would also be a viable method for manufacturing dry substance for determination of oral activity, though the requirement of a stabiliser in the process may limit its early use in development. How easy it is to produce nanocrystals does rely on the degree of crystalinity for milling and hydrophobicity and solubility, (not necessarily in water) for precipitation methods, meaning chemical differences in substances are still relevant.

This approach would be especially useful for class IV compounds if successfully combined with compounds or carriers that improve permeability,[40] so long as the formulation remained stable. These areas have been the focus of some promising research.[41][42][43]

If the substance was chosen as a lead compound, further development would be required to ensure stability of the parenteral solution, or to develop the nanosuspension into an oral delivery system or other dosage form. For example if this would not be an economic approach, due to excessive amounts of energy required for milling, either loading onto nanoparticles or into carriers such as liposomes or micelles are techniques that can be employed.

The reason for the increased Cs is described by the Kelvin,[35][37][21] Ostwald-Freundlich and Prandtl[34][36][21] equations.

For a system of droplets of liquid the Kelvin equation-

(2)

γ is the surface tension, p the vapour pressure of the droplets, p0 the normal vapour pressure of the liquid, Vm The molar volume of the liquid, r the radius of the spherical droplet, M the molecular weight, ρ the density and R and T and gas constant and temperature.

From this equation it is clear that smaller droplets should have an increased vapour pressure, due to their sharp curvature. Therefore the transition from the liquid to vapour phase becomes more favourable.

The difference in pressure is usually too small to have an effect[38][39] but as Defay and Prigogine[39] demonstrated, at smaller droplet sizes this begins to have a pronounced effect as shown below[39](Table 15.1. p. 221)

Table 1. The effect of curvature on the vapour pressure of water droplets.

Radius, um

P/P0

∞, plane

1.000

1.0

1.001

0.1

0.01

0.001

1.011

1.115

2.968

To transfer this model to the dissolution of solid drug substance to a liquid phase, dissolution pressure is equivalent to vapour pressure. Cs, the saturation solubility is an equilibrium between dissolving molecules, equivalent to dissolution pressure and recrystalising drug. This decrease in size moves the equilibrium and this increases the Cs.

The Ostwald-Freundlich equation also predicts this phenomena of increasing Cs-

(3)

C is the solubility, C∞ the solubility of the solid consisting of large particles, σ interfacial tension, V is the molar volume of the particle compound, R is the gas constant, T the absolute temperature, ρ density of the solid, and r is the radius.

A different explanation for this effect is similar to the reason that many polymorphs have different Cs, due to the high-energy surface of the nanoparticles created in nanosizing the thermodynamically stable microcrystals. Therefore the particles exist in a higher energy state than the bulk compound so solvation energy is correspondingly larger. Cs is also a function of interfacial tension and it follows that as particles become smaller interfacial tension and energy increase which increases Cs.[20][21]

2.2.2. Stabilisers

Stabilisers are usually required to prevent effects such as Ostwald ripening, due the high surface energy of the particles[1][11][17][18][20][24] and do this by providing a steric or ionic barrier to the process of agglomeration. This is important to the shelf-life and toxicological aspects of the drug as microparticles in the bloodstream can lead to blockages of capillaries.

Polymeric stabilisers prevent recrystalisation and agglomeration of particles by surrounding fine crystals and thus reducing surface area that can interact with other drug substance, but they can also hinder dissolution by forming a barrier to penetrating water molecules.[11][35]

There is still research to be done in the area of developing a systematic approach to selecting suitable stabilisers for each compound. Currently an empirical approach using the most commonly successful stabilisers is the prevalent method. Research lead by Lee[24] did however discover some trends to keep in mind when selecting suitable stabilisers. In general matching stabilisers with similar surface energies to high MW, low solubility drugs successfully produced stable nanosuspensions. In fact it has been further reported that for compounds of solubility of less than 1mg/ml the colloidal suspensions produced were found to be very stable with regards to heat and over time.[17]

2.2.3. Other Advantages

Nanoparticles lead to a decreased variability in bioavailability and uptake between fed/fasted states, [17][18][20][22] which is caused by the increased presence of bile salts and phospholipids.[7] This is due to the adhesiveness of nanoparticles sticking to the gut wall[18][20] caused by the increased amount of interactive forces between the drug and the surface. The magnitude of this force can be quantified[23] and is a factor in the trend of increased rate of adsorption.[1][17][22]

Formulating as nanocystals has advantages from a cost and regulatory standpoint as it can avoid dealing with the toxicological aspect of some of the more harsh excipients for poorly water soluble compounds used previously. These include organics and surfactants in high concentrations or require a high/low pH, meaning that for many of the formulations in which they were used they are the dose limiting factor. Nanoparticulate formulations generally can be tolerated in higher levels due to their removal.[17][20][22]

Nanoparticulate formulations also have an advantage over products which rely on amorphous drug substance for sufficient delivery. The drug substance should be in crystalline form which avoids the problem of the amorphous material restructuring itself back into its more thermodynamically stable form though as mentioned earlier effects such as Ostwald ripening must be accounted for.

An example of the increased efficacy of drug systems which nanosuspensions can produce was demonstrated in a study of the antiparasitic drug amphotericin B.[25] In which the nanosuspension formula was 28% more efficacious than either the liposomal or micronized (which showed no curative effect) formulations.

2.3. Liposomes and Polymeric Micelles

This however may be unfair to the potential of liposomes, they encompass the range of 80nm to 100μm[26][27][28][29] in diameter for 'traditional' liposomes and 1-1000nm in size for newer liposomes and solid-lipid-nanoparticles. Consequently they benefit from most of the properties of nanocrystals, with added targeting capacity though efficient drug loading can become a problem.

The ability to tailor the delivery vehicle for targeting purposes is a major driving force behind this area of research. The toxicological effects of many liposomal materials have been studied in depth, meaning many delivery systems begin with already approved materials as their carriers.[26][27][28][29][30][31][55]

They are however more sensitive to changes in the formulation and harder to fine-tune the targeting characteristics, due to stability issues, when compared to polymeric micelles.

Polymeric micelles also currently come in the same size range of 1-1000nm, are seen as cheaper to produce and the original main obstacle of the cytotoxicity of the polymers post release[32][33] has been overcome.[28][29][44][45][46][47][31][48][49][50]55] Liposomes however, generally have a longer circulation time in the blood as they have a small size and hydrophilic surface which help them to avoid elimination by the reticuloendothelial system. This has led to the development of biodegradable hydrophilic tailed polymers for use as carriers.[31][48][49][50][52][55]

There are advantages over 'traditional' liposomes. Micelles in particular can withstand higher physiological stress, have higher stability and there is more possibility of oral dosage forms. [28][33][44] Furthermore the ability to tailor the dosing pattern and the delivery to specific or difficult targets is more easily developed.[28][46][47][31][51]

2.3.1. EPR Effect and Passive Targeting

As Micelles/liposomes have a longer time spent in the bloodstream and fewer side effects when compared to many other methods of drug delivery, an advantage comes in the form of the enhanced permeability and retention (EPR) effect.[52][29][53][54] This is when the drug substance preferentially accumulates in places with 'leaky' vasculature i.e. tumours and infarcts. Polyethylene glycol micellular lipid derivatives and liposomes in general have been demonstrated to display this effect in numerous circumstances.[51][56][55] Another factor to take into account is that liposomes are eventually removed via macrophages in the liver and spleen, this has been used to passively target these cells.[57]

Using this passive targeting method it has been found, as discussed by both Torchilin[44] and Anatoly[52] in their reviews of the area is that tumour accumulation is directly related to the thickness of the particular tumours blood vessel wall. There is a maximum size of micelle that can penetrate this barrier. Smaller micelles do allow more efficient loading of poorly soluble drugs due to having shorter tails and a higher hydrophilic to hydrophobic phase ratio.[44] However if too low a size of micelle is employed some of the targeting benefit is lost as more locations of accumulation will be open to the more permeable micelle. Therefore with simple size control the liposomes/micelles can be targeted to different types of tumour/macrophage.

2.3.2. Active Targeting

Active targeting by the attachment of various ligands to micellular systems is also possible, for example many cancer cells or parasitic diseases over express certain receptors that can then be targeted,[79] for more detail the reader is directed to Torchilins[44] review on polymeric micelles for examples of attaching moieties to the polymers hydrophilic tail and Date et al[29] for a though review on the performance to date of using liposomes, polymeric nanoparticles and lipid nanoparticles in use against parasitic diseases such as malaria, highlighting the varied formulation approaches that can be taken in delivering to the same target.

It is important to remember that by no means are liposomes and polymeric micelles the only possible choice for nanocarrier systems, they have just been used to highlight some of the advantages and possibilities of the approach. For example gold nanoparticles have been shown to have promise for use in the treatment of pancreatic cancer.[92]

A further aspect to be considered is the ability to circumvent barriers to drug delivery if the substance is administered in the pure form. For example insulin cannot be administered orally due to adsorption in the digestive system, first pass metabolism and enzymatic breakdown, requiring the patient to be trained to administer, possibly involving multiple injections per day to effectively control.[19] This contributes to less than total patient compliance and so less than optimal management of the disease.[91]

Nanocarrier systems have been promising in delivering alternate dosage forms, nasal delivery[58] and oral[59] have both been shown to be effective for insulin delivery in animal models. This avoids invasive injections and minimises the risk of infections. This approach may also be useful in the delivery of other peptides and proteins.[60][61]

For examples on where different nano-delivery systems have been used and their effectiveness the reader is directed to a review by Couvreur et al.[28] For more on the targets and potential pathways of administration of nanocarriers the reader is directed towards reviews by Hillaireau et al[62] and Kingsley et al.[63]

3. Chemical Approaches

3.1. Salt Formation

Salts comprised a third of NME's approved by the FDA between 1995-2006.[6] They comprised 41% of NME's approved by the FDA in 2007-2010.

Screening the salts of lead candidates for improved activity and physiochemical properties is often one of the first steps in drug development and often part of the HTS process.[64][65][66][67]

It is the most common method for improving drug solubility and dissolution rate and the governing factors in salt selection and shall be discussed here.

There are many considerations to take into account when selecting a suitable counterion for a salt form. The size of the ion, crystal lattice energy, strength of anionic or cationic charge and pH of the saturated salt solution all contribute to stability and solubility.[10][68] This is illustrated in a study of various dichlofenac salts with structurally related amines in which the solubilities of the salts varied by over a factor of 100.[66]

3.1.1. Influence of pKa and pH

The first factor that should be taken into account is the pKa of the ionisable groups of the molecule,[69][70] and this compared to the pKa of potential counter ions, these are easily found in the literature. Guidelines for the difference in pKa required for a stable salt are 2 or 3 units.[67][9] The next is the pH solubility relationships of the acidic/basic drug and the pH solubility relationship of the salt form.

In their work Kramer and Flinn[71] demonstrated that from two pH Vs solubility curves, one of the free base and one of the salt, it is possible to determine the pH range in which the salt would exist and the pH range that the free acid/base would exist in solution. The intersection of these two curves is the pHmax, for bases above which the free base would exist in equilibrium with the solution and below, the salt. There will only be a stable salt formed if the counter ion can reduce the pH to below that of pHmax.[9]

As discussed by Serajuddin,[6] in his review on salt formation to improve drug solubility, that in terms of equilibria, this relationship can be modelled by the following equations for monobasic compounds -[71]

A base or salt dissolved in water -

(4)

BH+ is the salt form and B is the free base form. When at a given pH and saturated with free base, the solubility (St) -

(5)

Where 's' indicates the saturation species. A similar treatment for the salt as the saturation species gives -

(6)

For a base and its salt the solubility behavior above pHmax is described by (5) and below by (6). Only at pHmax can the salt and the free base coexist. Bogardus and Blackwood[93] suggested that at pHmax the solubilities of B and BH+ were equal, then solving the equations for pHmax derived that-

(7)

Thus in choosing a suitable salt both the solubility of the drug must be taken into account as well as if it forms a stable salt. It may be decided to develop the free base form at this point but the possibilities of multiple hydrates and their possible interconversion must be investigated.[67]

Analogous relationships have also been found for acids.[72][73]

These equations have been extended for both polyprotic and polybasic compounds and it can be determined from the graphical representation whether a counter ion is capable of forming a poly- salt.[9] This is useful as determining the multiple pHmax values of a compound, then comparing to possible counter-ion candidates, time and resources can be saved by eliminating ions too weak to produce the necessary pH shift to produce a desired salt or polysalt.

3.1.2. Common Ion Effect

Another consideration when selecting suitable counterions is the common ion effect. This is where the dissolution rate and solubility of the salt is lowered by the presence of other ions. For a basic, undissolved solid salt (BH+X-) below pHmax -[6][9]

(8)

[BH+]s is the salt solubility and [X-] is the counterion concentration.

From which follows the apparent solubility product (K'sp)-

(9)

When the counterion is not in excess [BH+]s = [X-] => solubility =√ K'sp

Under these conditions the solubility of the salt remains constant, but if a large excess of counterion is added, e.g. to reduce pH. Then the following relationship is seen -

(10)

As can be seen form this relationship, this effect is particularly pronounced for salts of already low solubility.

Hydrochloride salts are often chosen due to their high pKa values leading to almost always being able to produce a stable salt. But as the gastro-intestinal tract contains Cl- ions in the source of bile salts, the concentration of which increase after eating, [74] they can present a common ion effect. This is a major contributor to the occasional disparity in bioavailability between the fed and fasted states. Depending on the kinetics of the reaction the chloride ion can still exert an effect[75] on non-chloride salts if not as great an effect as on the chloride salt itself, possibly leading to the hydrochloride salt precipitating out if it is particularly poorly soluble. Due to this effect mesylate salts are becoming more common,[6][76] especially in oral dosage forms.

3.3.3. Other Considerations, Drawbacks and Further Development

Anderson and Flora[68] suggest that solubility generally increases with more highly charged counterions and decreases with larger ionic radius, due to the free energy contributions from the crystal lattice and solvation energies of the ions. The effect of counterion on salt solubility would depend on whether lattice or solvation energies are most sensitive to changes in salt structure.

The order of solubility required can also effect the selection of salt, for example in suspension products excessive solubility can lead to Ostwald ripening and so a lower solubility would be required. Very high or very low solubilities can have an effect on the production method e.g. wet granulation is a poor choice for very soluble compounds and this must be taken into account for industrial scale processes.

A salt of a drug is often more stable than the free acid/base and may have a significantly higher melting point. This means that production processes that the drug may not be able to undergo e.g. various milling processes for micro/nanosizing, may become viable routes for development.

When forming a salt you are often increasing the MW of the drug and therefore the active content of each dose decreases. This leads to larger doses being required and may require several doses to administer. This is less desirable from a patient standpoint. There is the tendency of salts of forming an increased number of hydrates and polymorphs[10][76] which may make stability for formulations a problem, forming hydrates with water bound exipients etc. and involve extra characterisation work. Of course a major limitation of salt formation is that if a molecule is a neutral, or weakly ionisable molecule, salt formation is not an option. There is an increasing amount of these types of drugs entering development, limiting its utility.

With the increasing trend in extremely poorly soluble drugs being developed the formation of a salt alone may not have a large enough effect to effectively solubilise the drug. In these cases a combined approach is the best course of action, e.g. Jain and Banerjees[77] work suggests that solid-lipid and chitosan nanoparticles would be an effective delivery vehicle for ciprofloxacin hydrochloride and Kim et al[78] was successful in attaining the required solubility of ziprasodone mesylate via cyclodextrin complexation

3.1. Prodrugs

A prodrug strategy, that is attachment of a promoiety to a drug to overcome a delivery barrier, be it adsorption, solubility or other factor, followed by a biotransformation back into the original drug at delivery, usually through some kind of enzyme cleavage, is a technique that has existed for years. For example aspirin is a prodrug of sodium salicylate, but it is not often utilised except when other strategies have been exhausted. It is however, receiving more attention due to the trends mentioned at the beginning of this review. For example one approach for delivery of type IV 'brick dust' molecules is to modify it so that it becomes a 'grease ball'[9][10] from which stage any further problems may be more easily addressed.

In the last three years 3 out of 79 NMEs approved by the FDA have been listed as prodrugs and there are probably more whose mechanism of action is actually via a prodrug route. This 3-4% is lower than previous years for example in the period of 2001 and 2002 they were 14%[82][83] Both do show that prodrugs are a notable portion of drugs released but they are far from dominant as a development method.

This may be attributed to prodrug methods only being considered as 'last resort' methods by development teams possibly leading to the somewhat irregular pattern of new prodrug approval. The case has been made for their inclusion at an earlier point in the development cycle see Stella's book and review on the area.[84][85]

There are pros and cons of this as a prodrug solution does add further complications of extra synthesis steps, having to create a pharmacokinetic profile of both it and its parent drug and metabolism studies of both as well as the toxicological dangers of not only the drug and prodrug but also of the released promoiety.

Of course if a drug is chosen for development and only later discovered to have one or more poor drugable properties the time and resources lost may be considerably more than the cost to develop a prodrug. The patent clock of a drug can also be extended using a prodrug, developing a suitable prodrug with some advantageous property over the parent and then encouraging physicians to prescribe it whilst the original patent is still running can maintain exclusivity for longer e.g. fosamprenavir,[86] or in fact reacquire market dominance as in the case of fosphenytoin.[87]

3. Conclusion

The best way to overcome the challenges in delivering poorly water soluble drugs is though a combination of the chemical and formulation routes previously described. Each method has its strengths and weaknesses and all the options should be evaluated before choosing a way to proceed. This approach allows very fine control of rate of adsorption, location of activity, bioavailability and controlled release. This is best illustrated with a recent novel example. Work by Vukomanovi´ et al[80][81] demonstrates a prolonged controlled release of an antibiotic and its prodrug from a nanocarrier system of poly(D,L-lactide-co-glycolide) (PLGA) spheres, linked by rod like particles of ceramic hydroxyapatite (HAp) forming a core/shell nanostructure. Picture would be useful One phase of the drug is dispersed within the spheres, with the second phase of the drug dispersed at the interface between the rods/spheres.

Using this system to deliver the antibiotic clindamycin and its prodrug clindamycin phosphate, they found that a high concentration could be achieved in vitro for 30 days, which for many antibiotics is a required part of treatment. Also the (PLGA) is biodegradable and the hydroxyapatite is bioreactive which promotes this degradation and could be tailored to control degradation and therefore release further. The degradation products of the PLGA are acidic[88] and this may have unwanted effects such as bone resorption.[89]This effect however can be stabilised with carbonated calcium phosphates[90] which react with the acidic degradation products to steady the microenvironmental pH by dissolution and re-precipitation of HAp and could also be used to futher control the degredation and therefore release of drug futher. This particular approach relies on the different physiochemical properties of the drug and prodrug for the preferred release schedule as well as the properties of the carriers for the kinetics of release. Early investigation of prodrugs of proposed extended release drugs could prove valuable if properties allow this kind of relationship for delivery.

A combinatorial approach marrying the most advantageous aspects of each method can yield unprecedented control over dosage, targeting and bioavilability. This allows choice in selecting drugs to develop that is still economical over a wider range of solubilities. This is becoming more prevalent and promises to be a powerful methodology, opening up new therapeutic routes and new uses and delivery forms for older drugs. Repeats a little.

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