Experimental study method, results and discussion
Three drugs were studied:
- Anti-inflammatory (A1) - very slightly soluble
- Bronchodilators (B1 and B2) - insoluble
General sample preparation
Approximately 30mg of drug were weighed accurately into a 30ml scintillation vial and approximately 20ml aqueous solution added gravimetrically. Each sample was tumbled for 24 hours on a Denley Spiramix 10 roller mixer. For each drug, it was found this was sufficient time to ensure wetting of the particles.
Initial visual observation (water only)
- A1 appreciably wetted
- B1 and B2 very hydrophobic (floated on top of liquid)
Tumbled overnight then visual observation (water only)
- A1 – partially precipitated on vial surface
- B1 and B2 dispersed after tumbling with no visual evidence of aggregation
Because A1 shows evidence of dissolution and precipitation, using water alone doesn't meet the universal vehicle objective.
Zeta potentials for each drug in water
The above table shows the zeta potentials for each of the drugs in water following tumbling for 24 hours.
Partially-soluble anti-inflammatory A1 possesses a modest positive zeta potential and also deposits in the same manner as in water only. It is highly unlikely that the zeta potential would be sufficient to provide adequate electrostatic stabilization.
Both insoluble bronchodilators, B1 and B2, exhibit unusually high negative zeta potentials. These would be expected to provide robust electrostatic stabilization.
Zeta potentials for A1 in aqueous KCl solution
The aggregation rate of the primary drug particles is influenced by the presence of electrolyte. Screening of the particle charge by the electrolyte leads to a general increase in aggregation rate at higher electrolyte concentrations.
The zeta potential remains nominally constant with increasing electrolyte concentration up to 10-3 mol dm-3. At higher concentrations, the samples exhibit significant aggregation which makes zeta potential measurement difficult.
Zeta potentials for B1 in various electrolyte solutions
Visually, bronchodilator B1 formed colloidally-stable dispersions in each electrolyte at low electrolyte concentrations (typically less than 10-3 mol dm-3). It was not possible to measure zeta potentials at or above 10-3 mol dm-3 electrolyte concentration due to aggregation.
The zeta potential data shown above suggest that the surface charge is unaffected by monovalent cations (K+) but is reduced by divalent cations (Mg2+).
The implication of these data is highly significant. They show that the dispersions aggregate at moderate electrolyte concentration (10-3 mol dm-3) even though the zeta potentials at lower electrolyte concentrations including zero are very high (-60mV). This is a very good example of why the rule-of-thumb mentioned previously regarding minimum zeta potential required for stability cannot be considered universal. For this reason, it is insufficient to measure zeta potential without additional characterization such as visual observation, laser diffraction and/or dynamic light scattering.
Surfactant study
The role of the surfactant is to adsorb onto the drug surface and, in so doing, further increase the electrostatic charge. The choice of surfactants permits a simple verification of this mechanism. Each surfactant has the same length hydrophobic tail, which will adsorb onto the particle. Only the head group varies. DTAB will adsorb to give positively charged particles; SDS will adsorb to give negatively charged particles; and DGP will adsorb without significantly altering the electrical nature of the drug surface. Hence, DTAB and SDS should act as good stabilizers whereas DGP should not. Furthermore, the presence of surfactant in solution will lower the surface tension between the drug and the water, enabling faster wetting of the drug. The rate of wetting was found to be sufficiently fast that, unlike the case for drug in water, tumbling overnight was not required.
Drug A1
Drug A1 becomes wetted very rapidly (few seconds) in the presence of the ionic surfactants (SDS and DTAB) and did not precipitate onto the vial surface after tumbling overnight.
The DTAB system remained opaque after standing for 24 hours.
Following 5 minutes ultrasonication, the SDS-based system appeared pearlescent. Upon agitation, birefringence was observed. Birefringence occurs due to long range ordering of the particles in at least one dimension, which affects the way in which light of different polarization is scattered. Streaming birefringence, as observed here, occurs when irregularly shaped microcrystalline particles (such as drug particles) align themselves to some degree in the fluid flow. Approximately 24 hours after the start of sedimentation (i.e., free standing), the vials were inverted to assess the nature of the sediments (shown photographically below).
During sedimentation, non-aggregated particles are able to form a closely packed structure in the sediment. Aggregated particles can only form loosely-packed sediments. The closely-packed sediments need more energy to be disrupted than the loose sediments, therefore the former remain predominantly intact at the bottom of the vial.
The table of zeta potentials below shows that the surfactants have the predicted effect on the particle charge, yielding very high zeta potentials for the ionic surfactants.
Drug B1
Drug B1 is wetted very rapidly in the presence of the ionic surfactants, especially in the case of SDS. DGP does not confer as much stability as the ionic surfactants. Inversion of the vials after a further 20 hours is shown photographically below.
The apparent strong adhesion of the drug/DTAB sediment is likely to be due to the electrical attraction between the positively charged surfactant-coated particles and the negatively charged glass. It is worth noting that the drug/DGP system does not form an adhesive sediment. It can be seen in the table of zeta potentials below that the drop in zeta potential in the presence of DGP (-43mV) compared to the drug in water alone (-65mV) suggests the adsorption of some of the surfactant onto the drug.
Drug B2
Drug B2 shows similar wetting behavior to B1. The main difference is in the appearance of the sediments after inversion (below).
Surfactant-dependent zeta potential for all drugs
Surfactant study size distributions
The three figures below show the particle size distributions for each drug-surfactant combination following sedimentation and redispersion.
In each case, measurements were made in triplicate via laser diffraction. Casual observation leads to these important conclusions:
Visual observation is a powerful tool to identify aggregated systems but below a threshold aggregate size, it cannot readily differentiate between high dispersed and weakly aggregated systems. A quantitative technique such as laser diffraction is necessary to distinguish the two states. For example, drug A1 with DTAB visually appears to redisperse well but the laser diffraction data show that aggregation is still present since the target size is ~2μm but the measured size is ~6μm.
Full redispersion of A1 is not possible with any of the surfactants.
Full redispersion of B1 and B2 is achieved with SDS. With DTAB, the particle size distribution is similar for the particles in water alone in spite of the very high zeta potentials.
None of the surfactants meet the criteria required for a universal vehicle.
Polymer study
For safe use in inhaled suspensions, it is believed that the molecular weight of stabilizing polymers should be kept below approximately 15,000. This constraint may preclude the use of many polymers. However, if the water-soluble block is partially ionic, then an electrostatic contribution will be added to the interparticle repulsion. Furthermore, because the large size of the particles will lead to the formation of fine, dilatant sediments, the extra electrostatic repulsion should help redisperse the particles. In general, polymer-stabilized particles will be redispersed more readily to primary particles than purely electrostatically-stabilized ones, including surfactant-stabilized systems.
Polymers used
Poly(vinyl pyrrolidone), PVP, was chosen as the homopolymers:
It is widely used in the food and pharmaceutical industries. For this work, low molecular weight PVP was used (nominal molecular weight = 15,000).
Poly(vinyl alcohol-co-vinyl acetate), abbreviated to PVA, was chosen as the random copolymer:
where n/(m+n) is typically 0.8 to 0.9. The different moieties are arranged randomly. PVA is widely used in the pharmaceutical, cosmetics, adhesives and paper industries. It is considered non-toxic by ingestion. For this work, low molecular weight PVA was used (nominal molecular weight = 9 to 10,000) with an 80% degree of hydrolysis.
Low molecular weight (5 to 10,000), block copolymer versions of Eudragit (a methacrylate-based high molecular weight random copolymer, widely used as a pharmaceutical excipient) were used:
where m/n = 3. A range of polymers were studied. Only the results for one of them are presented here (P07; molecular weight ~10,000).
A1 size distributions after redispersion
The figure above shows a comparison between formulations separately prepared with PVA, PVP and P07. Only P07 offers redispersion of particles back to their fully dispersed state. Limited zeta potential measurements were performed giving -10mV and +33mV for PVA and P07, respectively. This confirms the likely electrosteric stabilization in the presence of P07.
Block copolymer P07 with each drug
The figure above shows the post-sedimented and redispersed particle size distributions for each drug formulated separately with P07. The size distributions match the known size distributions for the drugs in their micronized powder state (not shown).
Visual appearance of sedimented samples of A1 using polymer P07
The preceding work used surfactant and polymer concentrations equivalent to approximately 15% of the drug by mass. One of the criteria to meet the study objective is that the chosen surfactant or polymer be effective at less than 10% of drug by mass.
Given the issues of dissolution and precipitation of A1, the effect of concentration of polymer P07 on the visual suspension properties of A1 dispersed into polymer solution. The photograph below shows the appearance after equilibration and standing for dispersions of A1 at polymer concentrations ranging from 0 to 15% of the drug by mass (increasingly linearly from left to right). Illumination was from behind.
The noteworthy observations are given below. Some of these are based on the original photographs and not necessarily evident in the above reproduction.
Significant amounts of unscattered transmitted light are seen in samples A to F indicating non-uniform dispersion of the drug.
A and B appear to have partially dissolved and reprecipitated onto the vial surface, as expected.
The heights of the sediments in C to F indicate strong aggregation due to insufficient polymer to fully cover the surface of the drug particles.
Samples G - K show no evidence of aggregation. This is supported by the diffuse appearance of the transmitted light. The heights of the sediments formed are significantly smaller than for the aggregated cases (C to F).
A minimum polymer concentration of 9% by mass of drug (G) is required to prevent aggregation based on visual observation.
Adsorption isotherms
Preparing samples for visual observation as a function of concentration of the dispersant is the same as the first step for determining adsorption isotherms. The figures below show such isotherms for drug B1 in the presence of a 5K block copolymer and, separately, a 10K block copolymer (P07) of the same type as used for the polymer study described earlier.
From the shape of the isotherm it is evident that both polymers adsorb strongly onto the drug and that once a monolayer of polymer has adsorbed, no more does. This represents classic strong adsorption. The amounts required to create one layer (monolayer coverage) are approximately 1.3mg m-2 and 2.5mg m-2 for the 5K and 10K molecular weight, respectively. It is common that the ratio of the monolayer amounts is the same as the ratio of the molecular weights. Furthermore, the monolayer coverage in both cases is achieved at equilibrium concentrations less than 0.2 mg ml-1. In these cases, this is less than 10% of drug by mass.
Similar observations were made for A1 and B2.
Concluding statement
Quantitative assessments of the adsorption behavior, particle size distribution after sedimentation and redispersion, and the applicability to each drug demonstrate that the use of P07 meets all the criteria required for a universal vehicle.