Comparison of Limulus Amoebocyte Lysate and Recombinant Factor C Assays for Endotoxin Detection in Four Human Vaccines with Complex Matrices

Endotoxins, heat-stable lipopolysaccharides from Gram-negative bacteria, are potential contaminants that can be introduced during manufacturing of pharmaceutical products, including vaccines. Parental pharmaceutical products undergo endotoxin testing because endotoxins are pyrogenic in humans and can induce severe physiological reactions. Currently, animal-derived Limulus amoebocyte lysate (LAL) assays are widely used. Assays using recombinant factor C (rFC), a nonanimal-derived reagent, have been proposed as alternatives. Some components in the matrices of pharmaceutical products can interfere with these assays. We compared two LAL- and two rFC-based assays for endotoxin detection in four complex human vaccine matrices. We showed that the results for the rFC-based assays were at least equivalent to those for the LAL-based assays, although the rFC-based assays were found to be adequate but slightly less suitable for one of the products that contained proteases as the methods used to inactivate the proteases reduced the assay performance. Likewise, LAL was adequate but less suitable for another product that contained glucans. The rFC assays offer a number of benefits, including compliance with the principles of the 3Rs, i.e., replacement, reduction, and refinement of animal testing by safeguarding animal welfare and promoting more ethical and sustainable use of animals for testing. After they are fully validated, as per the compendial requirements, they could be considered as suitable replacement assays for the detection of endotoxin in the manufacturing processes of pharmaceutical products. In summary, we demonstrated that both LAL and rFC assays are adequate for testing and releasing four vaccine products.

Airborne endotoxin from indoor and outdoor environments: effect of sample dilution on the kinetic Limulus amebocyte lysate (LAL) assay

Airborne endotoxins in occupational environments are a potential respiratory hazard to individuals. In this study, airborne endotoxins were collected using open-face and button aerosol samplers from inside animal housing units and downwind from agricultural production sites and a wastewater treatment plant. Filter extracts were then diluted to examine the effect of interfering substances on the kinetic Limulus amebocyte lysate (LAL) assay. In most cases, the overall endotoxin concentration was shown to decrease with increasing dilution up to 1000-fold, suggesting the presence of enhancing substances in the filter extracts. This dilution-dependent effect was most prominent in the open-face endotoxin samples, while button samples displayed little effect. Using a joinpoint regression model, it was determined that a dilution factor of 50 to 100 was generally sufficient to eliminate the presence of enhancing substances. After screening the data for dilution dependent effects, the airborne endotoxin concentrations were determined. The highest endotoxin concentrations, ranging from 2841 to 49,066 endotoxin units m(-3) of air, were found inside swine farrowing and finishing barns. Airborne endotoxin concentrations were 10- to 100-fold lower inside a dairy barn and downwind of other agricultural production sites and the wastewater treatment plant. Examination of dilution-dependent effects should be considered essential when utilizing the LAL assay, especially if values are to be used for regulatory purposes.

Setting a standard: the limulus amebocyte lysate assay and the assessment of microbial contamination on spacecraft surfaces

Historically, colony-forming units as determined by plate cultures have been the standard unit for microbiological analysis of environmental samples, medical diagnostics, and products for human use. However, the time and materials required make plate cultures expensive and potentially hazardous in the closed environments of future NASA missions aboard the International Space Station and missions to other Solar System targets. The Limulus Amebocyte Lysate (LAL) assay is an established method for ensuring the sterility and cleanliness of samples in the meat-packing and pharmaceutical industries. Each of these industries has verified numerical requirements for the correct interpretation of results from this assay. The LAL assay is a rapid, point-of-use, verified assay that has already been approved by NASA Planetary Protection as an alternate, molecular method for the examination of outbound spacecraft. We hypothesize that standards for molecular techniques, similar to those used by the pharmaceutical and meat-packing industries, need to be set by space agencies to ensure accurate data interpretation and subsequent decision making. In support of this idea, we present research that has been conducted to relate the LAL assay to plate cultures, and we recommend values obtained from these investigations that could assist in interpretation and analysis of data obtained from the LAL assay.

Bacterial endotoxin testing: a report on the methods, background, data, and regulatory history of extraction recovery efficiency

Since the mid-1970s the Limulus Amebocyte Lysate (LAL) assay has been used to test medical devices for bacterial endotoxins. The Association for the Advancement of Medical Instrumentation (AAMI) recently published a standard designated ANSI/AAMI ST 72: 2002, Bacterial Endotoxins--Test methodologies, routine monitoring, and alternatives to batch testing, which addresses LAL testing and associated issues. In order to perform the bacterial endotoxins test (BET), the test article must be extracted in an aqueous medium, with the extract being used as the test solution. In the early years of testing, and periodically throughout LAL test history, questions have arisen about validation of the extraction efficiency of endotoxins from medical devices. The AAMI Microbiological Methods Committee appointed a Task Group to thoroughly research the issue of extraction efficiency and to recommend whether validation of extraction efficiency is necessary for LAL testing of medical devices.

Microanalysis of nucleic acids using the limulus G test

The limulus G test has been used as a quantitative analysis of (1-->3)-beta-D-glucans, including schizophyllan (SPG) and curdlan. The present work extended the limulus G test to detect polynucleotide/SPG complexes. The complex showed an extremely sensitive response to the test, compared with SPG itself. The minimum concentration of the complex to show the response is almost 10-times as small as that of SPG itself, indicating the possibility to detect (1-->3)-beta-D-glucans or/and polynucleotides on the pico gram/ml scale.

Disposable TSM-biosensor based on viscosity changes of the contacting medium

The thickness shear mode (TSM)-sensor responds to changes of mechanical properties of the material contacting the surface of the sensor. One of the material properties is the viscosity of a liquid. Abiosensor based on the TSM-resonator for the detection of endotoxin has been developed. It exploits the viscosity-density change during the reaction of endotoxin with limulus amebocyte lysate (LAL). The effect of surface properties of the sensor has been investigated to achieve better output signals. It is shown that the sensor requires a hydrophilic surface to get a better coupling between the sensor and the LAL-endotoxin solution. The TSM biosensor is able to detect an endotoxin concentration as low as 100 fg/ml by using only 50-microl standard LAL solution. The disadvantages of reusable sensors, such as the contamination from previous measurement of endotoxin and the cost of the regeneration or reclining processes of the sensor, have been eliminated by using a cost effective disposable TSM-sensor.

[Replacement of the pyrogen-test as batch control test for the biological substances aprotinin and urokinase]

In the Pharmeuropa Vol. 10, No. 4, December 1998, the European Pharmacopoeia Commission published an appeal to replace the pyrogen test by the limulus test for a total of 10 biological substances. These substances included the active ingredients aprotinin and urokinase, contained in our sales products Antagosan and Actosolv. After successful validation of the limulus test, we calculated endotoxin limit concentrations for these and submitted these to the European Department for Quality of Medicines (EDQM). We expect - not least on the basis of the data submitted by ourselves - that the pyrogen test will be replaced in future as a quality control test by the limulus test in testing aprotinin and urokinase compounds for the presence of pyrogenic substances. This leads to a reduction in the number of rabbits used as opposed to the current pyrogen test for testing the stated compounds.

Evaluation of the applicability of the bacterial endotoxin test to antibiotic products

Fundamental conditions for applying the bacterial endotoxin test to antibiotic products were investigated so as not to affect the level of regulation by the rabbit pyrogen test. According to accuracy evaluation of test methods, the kinetic-turbidimetric and kinetic-colourimetric assays were shown to allow more accurate measurement and, therefore, more sensitive detection of interference to the bacterial endotoxin test than the gel-clot method. In total, 102 antibiotic products were evaluated on their interfering effect to show that the antibiotics could be categorized into three groups depending on intensity of the interference. Although the test was shown to be applicable even to the group showing the strongest interference, it was assumed to be crucial to use appropriate reagents and an accurate test method for avoiding approval of a pyrogenic product. Accordingly, lists of antibiotics are presented to provide limits of concentration for eliminating interference and endotoxin limits for approval to facilitate effective bacterial endotoxin tests.

Preliminary report on the results of the second phase of a round- robin endotoxin assay study using cotton dust

In an on-going endotoxin assay study, a two-part interlaboratory endotoxin assay study has been completed. The purpose of the study was to compare the variation in assay results between different laboratories, and, if the variation was high, to see if a common protocol would reduce the variation. In both parts of the study, membrane filters laden with the same approximate amount and type of cotton dust were sent for analysis to laboratories that "routinely" perform endotoxin analyses. First, each of these laboratories performed the analysis using the methodology common to its laboratory. In the second part of the study, membrane filters with cotton dust were again sent to the same laboratories where the analyses were performed as before but with a common extraction protocol. The preliminary results from the first phase of the study have been collected and showed that intra-laboratory variations were small, but large and significant interlaboratory variation was observed. The results were reported elsewhere. The preliminary results from the second part of the study consisting of the data currently collected are presented here. Again, intra-laboratory variations were small, but, also again, large and significant inter-laboratory variation was observed. However, in this part of the study, the range between the highest and lowest average results was narrower than in the first part of the study. Influence of the assay kit type was examined. The variation within assay kit type was small but significant differences in results were observed between assay kit types. The findings suggest that endotoxin concentration in samples can be ranked within laboratories, but not necessarily between laboratories. However, some of the variation between laboratories has been reduced by a common extraction protocol which suggests the possibility of further standardization that may lead to better comparability between laboratories.

Resolving liposomal inhibition of quantitative LAL methods

Bacterial endotoxin is composed of Lipid A as well as variable polysaccharide regions, and is capable of eliciting a pyrogenic response in vivo (1). NeXstar has developed a quantitative LAL assay at maximum sensitivity to provide assurance that its products are well below the endotoxin pyrogenic threshold. This test uses rigorous treatment of 1.0 mL liposome samples with sodium dodecyl sulfate, an anionic surfactant, or CHAPS, a zwitterionic surfactant to disrupt the lipids present in the liposome sample including endotoxin. This liposome disruption allows potentially undetectable endotoxin to become available to the LAL cascade for quantification and overcomes inhibition of the LAL test by liposomes.

Validation of the limulus amebocyte lysate (LAL) test for routine PET radiopharmaceuticals

The kinetic turbidimetric limulus amebocyte lysate test was validated as method for detecting endotoxins in short-lived radiopharmaceutical samples. Using this method, radiopharmaceuticals can be released for administration to humans after the test, without extensive loss of radioactivity. Inhibition or enhancement on the LAL results by the product samples were examined in more detail and eliminated.

A novel method for the continuous measurement of endotoxin concentration

Conventional limulus amebocyte lysate tests involved procedures to prevent contamination by atmospheric endotoxins. To address this problem, the authors have proposed a technique in which the sampling, reagent mixing, and reaction steps are carried out consecutively in a single tube. Since reagents do not come in contact with the atmosphere, the new technique promises stable determination of the concentration of endotoxins in dialysate fluid. An aqueous solution of endotoxin simulating dialysate fluid was sampled in a silicone rubber tube from a sterile infusion bag, then mixed with an indicator in the same tube. After reaction at 310 K, measurements were made of light absorbance at 405 nm and its linearity with endotoxin concentration was determined. Results showed a high degree of linearity (correlation coefficient of not less than 0.99) at endotoxin concentrations of 0-15 pg/ml. The time for the reaction was shortened to 12 min, in which case the response time was 15 min. It is suggested that this new test for determining endotoxin concentration using limulus amebocyte lysate reagent, in which all three steps--sampling, mixing, and reaction--proceed continuously in a single tube, offers higher reliability, greater ease of operation, and shorter response time than conventional tests.

Sensitive quantitation of endotoxin by enzyme-linked immunosorbent assay with monoclonal antibody against Limulus peptide C

Limulus peptide C, a 28-amino-acid fragment of coagulogen formed by the reaction of endotoxin with Limulus amebocyte lysate, was synthesized, and a monoclonal antibody against it was raised. A new microassay for endotoxin was developed, using this antibody in an enzyme-linked immunosorbent assay for generated peptide C-like immunoreactivity. A linear relationship between absorbance and endotoxin concentration was obtained. Control standard endotoxin in water could be detected to a level of 0.001 endotoxin unit per ml. The endotoxin levels in plasma samples from normal humans, rabbit, mice, and guinea pigs were generally found to be below the detection limit of 0.01 endotoxin unit per ml of plasma. The color and turbidity of specimens did not interfere with the assay. The consumption of Limulus amebocyte lysate in the assay was less than 5% of that in the gel-clot and chromogenic assays. With raw lysate, which was much more stable in solution than chloroform-treated lysate, the assay was still highly sensitive to endotoxin but was totally unresponsive to natural glucans. The monoclonal antibody cross-reacted with peptide C-like immunoreactivity generated in Tachypleus amebocyte lysate, which gave equal sensitivity in the endotoxin assay.

Specific assay for endotoxin using immobilized histidine and Limulus amebocyte lysate

The LAL test is inhibited or enhanced by many substances. To overcome these problems, we have developed a specific endotoxin assay method using an ultrafiltration unit, a fluorometric LAL reagent, and immobilized histidine (which is a specific adsorbent for endotoxins). This method is composed of two steps. The first step is the adsorption of endotoxins. Using immobilized histidine, endotoxins are quantitatively adsorbed on the adsorbent, and the adsorbed endotoxins are separated from LAL-inhibiting or -enhancing substances by the ultrafiltration unit. The second step is the reaction of adsorbed endotoxins with the LAL reagent. The endotoxins adsorbed on immobilized histidine are directly reacted with the LAL reagent in a filter cup and show enough activity for assay. The reproducibility and the accuracy of this method are high, and the recovery of endotoxins from a sample solution is more than 95%. The new endotoxin assay method using immobilized histidine can be utilized for the determination of endotoxins in a solution containing LAL-inhibiting or -enhancing substances such as amino acids and antibiotics instead of requiring employment of the more common gel-clot technique.

Statistical procedure for evaluating the sensitivity of Limulus amoebocyte lysate by using a reference lysate

A study was designed to estimate variability of the Limulus amoebocyte lysate test by comparing a reference lysate against itself. Three technicians performed parallel tests, i.e., titrated side by side, the contents of two vials of reference lysate on 4 different days using 24 vials of the United States reference lysate and 12 vials of the United States reference endotoxin. Each parallel test was replicated three times. From the sensitivity endpoints, ratios were calculated for each parallel test. These ratios were converted to the logarithm for estimating variability among technicians and among vials of endotoxin. By using the overall variability of log ratios, a statistical procedure was developed to evaluate the sensitivity of each lot of licensed lysate submitted to the Bureau of Biologics for release.