Inter‐ and intralaboratory variation of in vitro diffusion cell measurements: An international multicenter study using quasi‐standardized methods and materials

In vitro measurements of skin absorption are an increasingly important aspect of regulatory studies, product support claims, and formulation screening. However, such measurements are significantly affected by skin variability. The purpose of this study was to determine inter- and intralaboratory variation in diffusion cell measurements caused by factors other than skin. This was attained through the use of an artificial (silicone rubber) rate-limiting membrane and the provision of materials including a standard penetrant, methyl paraben (MP), and a minimally prescriptive protocol to each of the 18 participating laboratories. "Standardized" calculations of MP flux were determined from the data submitted by each laboratory by applying a predefined mathematical model. This was deemed necessary to eliminate any interlaboratory variation caused by different methods of flux calculations. Average fluxes of MP calculated and reported by each laboratory (60 +/- 27 microg cm(-2) h(-1), n = 25, range 27-101) were in agreement with the standardized calculations of MP flux (60 +/- 21 microg cm(-2) h(-1), range 19-120). The coefficient of variation between laboratories was approximately 35% and was manifest as a fourfold difference between the lowest and highest average flux values and a sixfold difference between the lowest and highest individual flux values. Intralaboratory variation was lower, averaging 10% for five individuals using the same equipment within a single laboratory. Further studies should be performed to clarify the exact components responsible for nonskin-related variability in diffusion cell measurements. It is clear that further developments of in vitro methodologies for measuring skin absorption are required.

Thermodynamic considerations of microgel swelling behavior

A simple but novel thermodynamic model is presented, based upon van't Hoff analysis, for the reversible swelling behavior of colloidal microgels. The swelling, as a function of temperature, of poly(N-isopropylacrylamide/N,N'-methylenebisacrylamide) as well as poly(N-isopropylacrylamide/vinylpyridine/N,N'-methylenebisacrylamide) and poly(N-isopropylacrylamide/acrylic acid/N,N'-methylenebisacrylamide) microgel dispersions in H2O and D2O has been studied by photon correlation spectroscopy (PCS). PCS data was used to obtain the hydrodynamic diameter and hence the volume of the microgels (before and after reconstitution following freeze-drying) as a function of temperature. The choice of standard reference states, for analyzing the data attained, is discussed, and the one selected is that of the volume of the microgels at 333 K in H2O. For all microgels examined the volume, at this temperature, is shown to be independent of solvent (H2O, D2O). The derived data has allowed the exploration of a novel thermodynamic approach to the study of the swelling behavior of the microgels. The constant volume, at 333 K, for each of the polymer systems constituting the microgels is suggested to be an intrinsic property of the polymers themselves.

UV-spectrophotometry study of membrane transport processes with a novel diffusion cell

A novel diffusion cell has been constructed which allows study of membrane diffusion processes without the need for sampling of the receiver compartment, that is highly sensitive and, being based around a diode array spectrophotometer also allows for continuous, real-time recording of multi-species concentration changes in the receiving compartment. The system is controlled to operate isothermally (via a Peltier control system) at temperatures between 15 and 85 degrees C. To examine the performance of this novel design, the transfer of tetracaine from a preparation in PEG 400 (20% tetracaine in PEG 400) has been studied. The results have been used to determine flux, lag time and related parameters. The performance of the novel cell is compared with results from traditional Franz cell diffusion studies.

The stability of benzoyl peroxide by isothermal microcalorimetry

Isothermal microcalorimetry may be used to determine kinetic and thermodynamic parameters for chemical reactions. This paper reports rate constants, determined as a function of temperature, and the activation enthalpy for the degradation of solid benzoyl peroxide as determined by isothermal microcalorimetry. Studies were conducted on aqueous suspension phase, solid benzoyl peroxide. In addition, supporting evidence is cited from work carried out in this laboratory on the solution phase degradation of benzoyl peroxide using UV-visible spectrophotometry. The activation energy obtained by microcalorimetry was E(a)=137.8+/-6.6 kJ mol(-1) and the activation energy obtained from UV-visible spectrophotometry was E(a)=112.7+/-4.2 kJ mol(-1).

The stability of benzoyl peroxide formulations determined from isothermal microcalorimetric studies

Recent developments in the analysis of microcalorimetric data output allow the possibility of determining both thermodynamic and kinetic parameters for complex reaction systems. Such experiments routinely take around 50 h, hence qualifying for the description rapid. The methods have earlier been applied to a study of the stability of benzoyl peroxide itself in aqueous suspension. This paper reports the results of isothermal microcalorimetric study of the stability of benzoyl peroxide in the presence of a wide range of excipients and in formulated materials. The results are shown to assist in formulation design, are achieved rapidly and are derived from direct experimental study of the complex systems themselves. That is, no ancillary information is required nor are the studies invasive or destructive.

A microcalorimetric comparison of the anti-Streptococcus mutans efficacy of plant extracts and antimicrobial agents in oral hygiene formulations

AIMS

This study aimed to evaluate the efficacy of 'natural' putative antimicrobial agents against Streptococcus mutans and to compare these with synthetic agents using the flow microcalorimeter. Streptococcus mutans is one of the oral pathogens responsible for dental caries.

METHODS AND RESULTS

Traditional microbiological techniques are invasive and destructive unlike flow microcalorimetry. This rapid technique was used to continuously monitor the power output (bioactivity) of Strep. mutans with reproducibility, precision and accuracy. The antibacterial agents found in oral hygiene products and all the natural agents tested showed anti-Strep. mutans ability.

CONCLUSION

In this study microcalorimetry identified agents that had a biological effect and quantified the rate of kill achieved enabling four broad categories of antimicrobial agent to be defined.

SIGNIFICANCE AND IMPACT OF THE STUDY

Microcalorimetric data are a better indication of antimicrobial efficacy than merely determining concentrations at which an antimicrobial agent is bacteriostatic or bactericidal.

QSAR based on biological microcalorimetry

In this paper we describe a QSAR based on biological microcalorimetry for a set of antimicrobial hydrazides acting against Saccharomyces cerivisiae and Escherichia coli. Results show that an extrathermodynamic relationship exists based upon partitioning (log P(TA)) and microcalorimetrically measured biopotencies using the same cell systems. Moreover, the extrathermodynamic relationship between drug potencies for these two cell systems shows that both cellular systems appear to behave in the same way with respect to the importance of partitioning. This means that the same set of congeneric compounds experience a similar environment in the two systems. This represents a lateral validation of the method and discloses the validity of the QSAR model.

Influence of Alkyl Chain Length and Structure on the Extraction of Copper(II) from Aqueous Acid by 5-Alkyl-2-hydroxybenzaldoximes in Hydrocarbon Solvents: Diffusion Coefficients of Extractants and Their Complexes

Measurement of the kinetics of extraction of copper(II) by 19 different 5-alkyl-2-hydroxybenzaldoximes (alkyl is C7H15 and C9H19) from acidic (hydrogen sulfate buffer) aqueous solution in the range pH 3.4-2.0, into 20% v/v toluene in n-hexane is reported. The lowering of the interfacial tension has been measured in some cases. The solubility of the extractants in water has been measured spectrophotometrically. A modified Taylor-Aris dispersion technique has been used to measure the diffusion coefficients in n-hexane and Orfom SX7 of some of the extractants and of the copper(II) complexes which they form. At low initial bulk concentrations (<10 mol m-3) of the extractants, the rate of extraction is controlled by the rate of diffusion of the extractant from the bulk to the organic-aqueous interface. The diffusion coefficient is related to the steric bulk of the alkyl substituent. At higher initial bulk concentrations (200 mol m-3) of the extractants, when the interface is saturated, the rate of extraction is inversely related to the apparent steric bulk of the alkyl substituent. Copyright 1999 Academic Press.

Pharmaceutical microcalorimetry: applications to long-term stability studies

Calorimetry has been a mainstay of stability analyses for some time in the form of differential scanning microcalorimetry (DSC). This technique exploits high (relatively) temperature studies of pure materials and of formulations to accelerate any degradation or interactions. The behaviour of the material at storage or ambient conditions is then estimated via extrapolation from the Arrhenius equation. Recent developments in isothermal microcalorimetry allow the direct determination of both kinetic and thermodynamic parameters for long, slow reactions from studies conducted at appropriate temperatures and under designated environmental control (pH, pO2, RH etc.). This review introduces the kinetic analysis of microcalorimetric data and, through selected examples, shows applications of the method.

An acrylic ampoule represents a denture surface in a microcalorimetric biofilm problem

An acrylic microcalorimetric ampoule with inserts represented a denture surface in a Streptococcus mutants biofilm study. The biofilm of micro-organisms did not contain as many micro-organisms as those previously studied in planktonic suspension. A more sensitive microcalorimeter (Thermometric's thermal activity monitor) was required to detect the metabolism of S. mutants adhered to an engineered acrylic flow-through ampoule. Biofilm heat outputs were obtained in the acrylic ampoule following standardized procedures with cryopreserved aliquots of a homogeneous suspension of S. mutants.

Determination of receptor-bound drug conformations by QSAR using flexible fitting to derive a molecular similarity index

Results are presented for a QSAR analysis of bisamidines, using a similarity index as descriptor. The method allows for differences in conformation of bisamidines at the receptor site to be taken into consideration. In particular, it has been suggested by others that pentamidine binds in the minor groove of DNA in a so-called isohelical conformation, and our QSAR supports this suggestion. The molecular similarity index for comparison of molecules can be used as a parameter for correlating and hence rationalising the activity as well as suggesting the design of bioactive molecules. The studied compounds had been evaluated for potency against Leishmania mexicana amazonensis, and this potency was used as a dependent variable in a series of QSAR analyses. For the calculation of similarity indexes, each analogue was in turn superimposed on a chosen lead compound in a reference conformation, either extended or isohelical, maximising overlap and hence similarity by flexible fitting.

Microcalorimetric studies of the initial interaction between antimycobacterial drugs and Mycobacterium avium

To further our understanding of the mechanisms by which ethambutol potentiates the effect of other antimycobacterial drugs on mycobacteria we have studied the initial physico-chemical interaction between ethambutol and the Mycobacterium avium cell envelope using batch reaction microcalorimetry. When strains of M. avium were exposed to ethambutol an immediate endothermic reaction was recorded. When the M. avium cells were pre-treated with ethambutol this strongly affected the initial interaction between streptomycin and the bacterial cell surface. When the M. avium cells were simultaneously exposed to a combination of ethambutol and streptomycin an altered initial interaction with streptomycin was seen. These data suggest that ethambutol may potentiate the effect of other antibacterial drugs on M. avium by increasing cell wall permeability.

Flow microcalorimetric assay of antibiotics--II. Neomycin sulphate and its combinations with polymyxin B sulphate and zinc bacitracin on interaction with Bacillus pumilus (NCTC 8241)

A flow microcalorimetric assay for Neomycin has been developed which is monitored through interaction of the antibiotic with Bacillus pumilus as the test organism. The assay has better reproducibility (relative standard deviation 2.3%) and is more sensitive than conventional microbiological bioassay (0.5-2 micrograms ml-1). The effects of combinations with zinc bacitracin, with polymyxin B sulphate, and with both zinc bacitracin and polymyxin B sulphate (both in equimolar proportions), and in those proportions present in the commercial preparation TrisepR (ICI, Macclesfield, UK) have also been investigated. Synergy was observed for the combinations of Neomycin with the other two antibiotics in binary mixtures at the relative proportions found in TrisepR. The addition of all three antibiotics at the levels used in TrisepR did not show synergy. However, addition of all three antibiotics at equimolar concentrations did show synergy. It is suggested that microcalorimetry may be useful in in vitro experiments for exploring the relative proportions required for maximal effect in antibiotic combinations.

Flow microcalorimetric assay of antibiotics--III. Zinc bacitracin and its combinations with polymyxin B sulphate and neomycin sulphate on interaction with Micrococcus luteus

A flow microcalorimetric assay for zinc bacitracin has been developed which has better reproducibility (relative standard deviation less than 2%) and sensitivity (0.02 micrograms ml-1) than conventional microbiological assays, and requires an assay time of between 7.5-9 h. The assay is not suitable for zinc bacitracin determinations in the presence of equimolar concentrations of polymyxin B sulphate or neomycin sulphate, or of these antibiotics in the proportions in which they occur in the commercial preparation Trisep (ICI, Macclesfield, UK).

Flow microcalorimetric assay of antibiotics--IV. Polymyxin B sulphate, neomycin sulphate, zinc bacitracin and their combinations with Escherichia coli suspended in buffer plus glucose medium

Flow microcalorimetric assays for polymyxin B sulphate and neomycin sulphate have been developed using Escherichia coli as the test organism, suspended in a glucose plus buffer medium. These assays have a better reproducibility (relative standard deviations 3.2 and 2.0%, respectively), and require a shorter time (1 h including time required for preparation of the calorimeter) than do conventional microbiological assays, but are not as sensitive. It is suggested that a screening programme might produce a small group of more suitable (i.e. more sensitive) test organisms, which could be used to develop rapid and reproducible flow microcalorimetric assays for a wide range of antibiotics by the procedure described. The effect of combinations of polymyxin B sulphate, neomycin sulphate and zinc bacitracin (the antibiotic components of the commercial preparation Trisep, ICI, Macclesfield, UK) on the power output of cells suspended in glucose plus buffer medium is also reported. In defined combinations, the effects of neomycin sulphate appeared to be exerted before those of polymyxin B sulphate.

Flow microcalorimetric assay of antibiotics--I. Polymyxin B sulphate and its combinations with neomycin sulphate and zinc bacitracin on interaction with Bordetella bronchiseptica (NCTC 8344)

A flow microcalorimetric assay for polymyxin B sulphate has been developed which has a better reproducibility (relative standard deviation less than 3%) and sensitivity (0.35 micrograms ml-1) than conventional microbiological assays, and requires an assay time of ca. 4.5 h. The combinations with zinc bacitracin, with neomycin sulphate, and with both zinc bacitracin and neomycin sulphate indicate antagonism between these antibiotics upon interaction with Bordetella bronchiseptica (NCTC 8344). The combinations of all three antibiotics assayed were: (1) equimolar proportions; and (2) those proportions present in the commercial preparation TrisepR (ICI, Macclesfield, UK).

A thermodynamic analysis of the Collander equation and establishment of a reference solvent for use in drug partitioning studies

A thermodynamic analysis of the Collander equation, ln PI = a + b ln PII (I and II refer to two different partitioning systems with partition coefficients PI and PII, respectively), is given and applied to three forms of correlation. The intercept, a, is shown to have no general fundamental significance whereas the slope, b is shown to reflect differences in non-aqueous solvent properties; b is also shown to be of use in scaling solvent behaviour to select solvents which closely represent biological membrane properties for use in partitioning studies. Laboratory and literature data are subjected to the analysis.

A vacuum microbalance technique for studies on the wettability of powders

A vacuum microbalance technique has been used to evaluate the wettability of powders. The rate of uptake, and equilibrium weight of uptake, of water vapour onto outgassed powder samples, of differing wettabilities, were determined at different known temperatures. Standard techniques of data analysis were used to establish the enthalpy, entropy and Gibb's free energy of the activation and adsorption processes. The values obtained appeared to be in the correct order of magnitude and those for activation clearly reflected the relative wettability of the powders. Tests for compensation demonstrated that with the exception of phenobarbitone, there was probably a common mechanism producing the adsorption.

Kinetics of utilization of organic substrates by Mycoplasma mycoides subsp. mycoides in a salts solution: a flow-microcalorimetric study

The metabolism of various organic substrates by suspensions of Mycoplasma mycoides subsp. mycoides in a salts solution was followed by microcalorimetry. Enthalpy changes associated with metabolism were in good agreement with theoretical values. Substrate utilization showed Michaelis kinetics, allowing saturation constants (Km) and maximum specific rates of substrate utilization (Vmax) to be determined. In cells grown on a complex medium containing glucose, Km values were: glucose, fructose, N-acetylglucosamine, glycerol and pyruvate, less than 5 microM; lactate, 20 microM; glucosamine, 130 microns, and mannose, 1 mM. Values of Vmax for glycerol, pyruvate and lactate were similar and approximately twice those for glucose, mannose, glucosamine and N-acetylglucosamine; Vmax for fructose was one-quarter of that for glucose. In cells grown on complex medium in which glucose was replaced by mannose, glucosamine or N-acetylglucosamine, Vmax and Km for the respective growth sugars and for glucose were not significantly affected. However, in cells grown in the presence of fructose, Vmax for fructose increased to the value observed for glucose. It is suggested that M. mycoides is adapted to, and is constitutive for, the utilization of a single sugar (glucose), and a single amino sugar (N-acetylglucosamine), but that in the presence of fructose a fructose-utilizing pathway is induced.

The effect of chemical modification of Saccharomyces cerevisiae on electrophoretic mobility, cell-wall structure and amphotericin B uptake

Saccharomyces cerevisiae NCYC 239 suspended in solutions of NaCl showed two distinct plateaus in plots of electrophoretic mobility vs. pH, corresponding to pKa values of approx. 2 and 5. This is in contrast to cells suspended in buffer where only a single pKa (4) can be determined. Modification of cells with KI/I2 or nitrous acid led to altered electrophoretic mobility, indicating the presence of sulphydryl and amino groups, respectively, in the yeast cell surface, whereas uranyl nitrate modification had little effect, suggesting phosphate groups to be absent. Electron micrographs showed visible effects of KI/I2 and nitrous acid modification on cell membrane structure, and in these modified cells amphotericin B uptake was rapid. It is suggested that diffusion through the cell wall is the rate-limiting step for amphotericin B uptake. An activation energy of 20 kJ X mol-1 was determined for uptake of amphotericin B by unmodified cells.

Study of the interaction of Saccharomyces cerevisiae with glucose by particle microelectrophoresis

Saccharomyces cerevisiae NCYC 239 in the presence of glucose at temperatures under 303 K shows a time-dependent lowering of electrophoreric mobility v. At temperatures above 303 K, this time-dependent change in v is in the direction of increased mobilities. Cells suspended in buffer indicate a surface pKa of less than 4, whereas for cells suspended in buffered glucose it is impossible to derive a surface pKa. A kinetic study of the interaction of S. cerevisiae with glucose as a function of temperature allows calculation of an activation energy of 140 kJ X mol-1 for the combined processes of (i) uptake of glucose onto the cell wall, (ii) transfer through the cell wall and membrane, and (iii) the establishment of a steady glucose flux through the wall and membrane.

Enthalpies of solution of a series of m-alkoxy phenols in water, n-octanol and water-n-octanol mutually saturated: derivation of the thermodynamic parameters for solute transfer between these solvents

Enthalpies of solution in both water and n-octanol have been determined for a series of phenols, in a microcalorimeter modified for titration. The thermodynamic data for solution in n-octanol and in water, as well as for transfer between the two, have been calculated from the free energies of transfer, based upon partition coefficients, and the free energies of solution, based upon solubility data. The results are discussed in terms of solute-solvent interactions in both phases and are used to assess, critically, the use of n-octanol as lipid-like solvent in QSAR studies.

Oscillations in some linear free energy relationships derived from partition coefficients of phenols between octanol and water

In the partition of some resorcinol alkyl ethers between water and 1-octanol, the values of delta Gtrs do not increase in a regular way. Odd and even chain alkyl compounds show different, regular increases in delta Gtrs for addition of each methylene group. The unrecognized occurrence of this phenomenon in earlier data is pointed out, and its possible significance in medicinal chemistry is discussed.

Microcalorimetry as an immunological tool: preliminary studies with beef heart mitochondrial adenosine triphosphatase and its antiserum

The calorimetric technique was applied to a complex immunological system based upon purified beef heart mitochondrial adenosine triphosphatase (ATPase). Whole serum containing antibodies to the ATPase was obtained from a rabbit and used to study the antigen-antiserum interaction in comparison with control serum pooled from 50 non-immune rabbits. The results suggest that the immunological interaction may be accompanied by complex processes which were not detectable by other immunological methods. It was shown that kinetic analysis of the calorimetric data may resolve an apparently complex process of interaction into different component stages of reaction. The advantages of this type of application of calorimetry are discussed.

Flow microcalorimetric bioassay of polyene antibiotics: interaction with growing Saccharomyces cerevisiae

Microcalorimetric investigation of the interaction of polyene antibiotics with mid-exponential cells of a growing culture of Saccharomyces cerevisiae has been used as the basis of a bioassay procedure. The assay is rapid, sensitive and reproducible. The results are compared to classical assays and potency ranking orders.

The measurement of phenol coefficients by flow microcalorimetry

Microcalorimetric bioassay procedures have been applied to the determination of phenol coefficients and dilution coefficients. The derived values are compared with those from a standard AOAC test procedure. This microcalorimetric results indicate close agreement with AOAC values, good reproducibility (+/- 2%), rapidity (30 min per test), potential for automation and the determination of in-use dilutions of disinfectants.

Quantitative structure-activity relationships: the Van't Hoff heats of transfer of resorcinol monoethers from water to n-octanol

The partition coefficients of resorcinol and some resorcinol monoalkyl-ethers from water to n-octanol have been measured as a function of temperature. These data are used to evaluate delta G, delta H and delta S for the solute transfer process. The data suggest direct calorimetric measurement of these parameters should be performed. Correlation with calorimetrically determined 'biological response' will permit a more detailed and precise commentary upon the quantitative structure-activity relationships for these compounds.

Antibiotic bioassay by flow microcalorimetry

A rapid, sensitive microcalorimetric bioassay of good reproducibility has been developed for the antibiotics penicillin-G, carbenicillin, ampicillin and cefoxitin. The use of growing organisms as responding agents allows discussion of the results for systems which may involve biological fluids.

Antibiotic sensitivity testing by flow microcalorimetry

The proposed flow microcalorimetric method for the diagnosis of bacteriuria has been extended to include antibiotic sensitivity testing. Sensitive organisms rapidly (4-8 min) show thermal responses to the added antibiotics over the normal range of concentrations (1 x, 2x, MIC value).

A comparative study of the microbiological assays currently available for nystatin raw material

The classical agar diffusion and turbidimetric methods of assay for nystatin are compared with the more recently documented assays for this antibiotic which depend upon physicochemical measurement of the response of micro-organisms. Liquid nitrogen stored inocula were used throughout. It is concluded that the newer methods of assay are as reproducible and reliable as the agar diffusion and turbidimetric methods and that they are generally more sensitive. The choice between the assay methods compared can thus be based on speed, cost and sample through-put.

The application of cryobiology to the microbiological assay of nystatin

Improvements in the reproducibility of nystatin agar diffusion assays have been achieved by the use of liquid nitrogen stored inocula and deep frozen standard stock solutions. The overall percentage variability of the assay has been reduced from over 5% with daily prepared standards and inocula to around 1% with a frozen inocula and to 0.6% with a combination of frozen inocula and standards. The implications of these improvements in the standardization of nystatin assays, and microbiological assays generally are discussed.

Characterisation and metabolic studies of Saccharomyces cerevisiae and Kluyveromyces fragilis by flow microcalorimetry

The use of microcalorimetry in the routine identification of microorganisms is critically discussed and assessed. By use of flow microcalorimetric studies on Saccharomyces cerevisiae and Kluyveromyces fragilis the role of physical parameters and that of oxygen tension are discussed. The conclusion reached is that identification of microorganisms by microcalorimetry and subsequent discussion of metabolic events revealed by the thermogram, except under restrictive conditions, is inappropriate. However flow microcalorimetry, in contrast to batch microcalorimetry which has been used in the published material on microorganism identification, may allow characterization of yeasts suitable for particular industrial processes.

In vitro studies of amphotericin B in combination with the imidazole antifungal compounds clotrimazole and miconazole

The clinically important polyene antibiotic amphotericin B, in combination with two antifungal imidazole compounds, clotrimazole and miconazole, was studied in vitro. With use of results of cytoplasmic leakage, metabolic heat output, and minimal inhibitory concentration studies, a definite antagonistic response was demonstrated. It is suggested that, if combined antifungal drug therapy is clinically indicated, the drug combination be tested against the isolate by the simple technique of measuring cytoplasmic leakage or by the more elaborate method of flow microcalorimetry.