The bacterial endotoxins test (BET) is a test to detect or quantify endotoxins from Gram-negative bacteria using amoebocyte lysate from the horseshoe crab (Limulus polyphemus or Tachypleus tridentatus). There are three methods for this test:
- Method A:- The gel-clot technique, which is based on gel formation;
- Method B:- The turbidimetric technique, based on the development of turbidity after cleavage of an endogenous substrate;
- Method C:- The chromogenic technique, based on the development of color after cleavage of a synthetic peptide-chromogen complex.
APPARATUS:- Depyrogenate all glassware and other heat stable materials in a hot air oven using a validated process. A commonly used minimum time and temperature is 30 minutes at 250°C. If employing plastic apparatus such as microplates and pipet tips for automatic pipetters, use apparatus shown to be free of detectable endotoxin and which does not interfere in the test.
REAGENTS AND TEST SOLUTIONS
- Amoebocyte lysate:- A lyophilized product obtained from the lysate of amebocytes (white blood cells) from the horseshoe crab (Limulus polyphemus or Tachypleus tridentatus). Amebocyte lysate reacts to some β-glucans in addition to endotoxins. Amebocyte lysate preparations which do not react to glucans are available: they are prepared by removing the G factor reacting to glucans from amebocyte lysate or by inhibiting the G factor reacting system of amebocyte lysate and may be used for the endotoxin testing in the presence of glucans.
- Lysate TS:- Dissolve amebocyte lysate in water BET or in a buffer recommended by the lysate manufacturer, by gentle stirring. Store the reconstituted lysate, refrigerated or frozen, according to the specifications of the manufacturer.
- Water BET:- Water for injections or water produced by other procedures that shows no reaction with the lysate employed, at the detection limit of the reagent.
- Preparation of Standard Endotoxin Stock Solution:– A Standard Endotoxin Stock Solution is prepared from an endotoxin reference standard that has been calibrated against the International Standard for endotoxins. Follow the specifications in the package leaflet and on the label for preparation and storage of the Standard Endotoxin Stock Solution. Endotoxin is expressed in International Units (IU) of endotoxin.
Note:- One International Unit (IU) of endotoxin is equal to one Endotoxin Unit (EU).
Preparation of Standard Endotoxin Solution:- After mixing the Standard Endotoxin Stock Solution vigorously, prepare appropriate serial dilutions of Standard Endotoxin Solution, using water BET. Use dilutions as soon as possible to avoid loss of activity by adsorption.
Preparation of sample solutions:- Prepare sample solutions by dissolving or diluting drugs using water BET. Some substances or preparations may be more appropriately dissolved or diluted in other aqueous solutions. If necessary, adjust the pH of the solution to be examined (or dilution thereof) so that the pH of the mixture of the lysate TS and sample solution falls within the pH range specified by the lysate manufacturer, usually 6.0 to 8.0. The pH may be adjusted by the use of acid, base, or suitable buffer as recommended by the lysate manufacturer. Acids and bases may be prepared from concentrates or solids with water BET in containers free of detectable endotoxin. Buffers must be validated to be free of detectable endotoxin and interfering factors.
DETERMINATION OF MAXIMUM VALID DILUTION:- The Maximum Valid Dilution (MVD) is the maximum allowable dilution of a sample at which the endotoxin limit can be determined. Determine the MVD from the following equation:
Endotoxin Limit:- The endotoxin limit for parenteral preparations, defined on the basis of dose, equals K/M, where K is a threshold pyrogenic dose of endotoxin per kg of body weight, and M is equal to the maximum recommended bolus dose of product per kg of body weight. When the product is to be injected at frequent intervals or infused continuously, M is the maximum total dose administered in a single hour period.
Note: The endotoxin limit depends on the product and its route of administration and is stated in the individual monograph. Suggested values for K are:
– intravenous route: K = 5 IU endotoxin per kg body weight;
– intravenous route for radiopharmaceuticals: K = 2.5 IU endotoxin per kg body weight;
– intrathecal route: K = 0.2 IU endotoxin per kg body weight.
The endotoxin limit for parenteral preparations is specified in units such as IU/ml, IU/mg, IU/Unit of biological activity, etc., in the individual monograph.
Concentration of sample solution:-
– mg/ml in the case of endotoxin limit specified by weight (IU/mg)
– units/ml in the case of endotoxin limit specified by unit of biological activity (IU/Unit)
– ml/ml when the endotoxin limit is specified by volume (IU/ml)
λ: the labeled lysate sensitivity in the gel-clot technique (IU/ml) or the lowest concentration used in the standard curve for the turbidimetric or chromogenic techniques.
METHOD A: GEL-CLOT TECHNIQUE:- The gel-clot technique is for detecting or quantifying endotoxins based on clotting of the lysate TS in the presence of endotoxin. The minimum concentration of endotoxin required to cause the lysate to clot under standard conditions is the labeled sensitivity of the lysate TS. To ensure both the precision and validity of the test, perform the tests for confirming the labeled lysate sensitivity and for interfering factors as described below under Preparatory testing.
Test for confirmation of labeled lysate sensitivity:- Confirm in four replicates the labeled sensitivity, λ, expressed in IU/ml of the lysate prior to use in the test. The test for confirmation of the lysate sensitivity is to be carried out when a new lot of lysate is used or when there is any change in the test conditions which may affect the outcome of the test. Prepare standard solutions having at least four concentrations equivalent to 2λ , λ, 0.5λ and 0.25λ by diluting the Standard Endotoxin Stock Solution with water BET.
Mix a volume of the lysate TS with an equal volume of one of the standard solutions (such as 0.1 ml aliquots) in each tube. When single test vials or ampoules, containing lyophilized lysate are employed, add solutions of standards directly to the vial or ampoule. Incubate the reaction mixture for a constant period according to directions of the lysate manufacturer (usually at 37±1°C for 60 ± 2 minutes), avoiding vibration. Test the integrity of the gel for tests carried out in tubes, take each tube in turn directly from the incubator and invert it through approximately 180 degrees in one smooth motion. If a firm gel has formed that remains in place upon inversion, record the result as positive. A result is negative if an intact gel is not formed.
The test is considered valid when the lowest concentration of the standard solutions shows a negative result in all replicate tests.
The endpoint is the lowest concentration in the series of decreasing concentrations of standard endotoxin that clots the lysate. Determine the geometric mean endpoint concentration by calculating the mean of the logarithms of the endpoint concentrations of the four dilution series, take the antilogarithm of this value, as indicated in the following formula:
Σ e = the sum of the log endpoint concentrations of the dilution series used
f = the number of replicate test tubes
The geometric mean endpoint concentration is the measured sensitivity of the lysate (IU/ml). If this is not less than 0.5λ and not more than 2λ, the labeled sensitivity is confirmed and is used in tests performed with this lysate.
Test for interfering factors:- Usually prepare the solutions (A-D) in Table 1, and perform the inhibition/enhancement test on the sample solutions at a dilution less than the MVD not containing any detectable endotoxins, operating as described above under Test for confirmation of labeled lysate sensitivity.
The geometric mean endpoint concentrations of B and C solutions are determined by using the formula described above under Test for confirmation of labeled lysate sensitivity.
The test for interfering factors must be repeated when any condition changes which is likely to influence the result of the test.
– Solution A : a sample solution of the preparation under test that is free of detectable endotoxins
– Solution B : test for interference
– Solution C : control for labeled lysate sensitivity
– Solution D : negative control of water BET.
The test is considered valid when all replicates of solutions A and D show no reaction and the result of solution C confirms the labeled sensitivity.
If the sensitivity of the lysate determined in the presence of solution B is not less than 0.5 and not greater than 2λ, the sample solution does not contain factors which interfere under the experimental conditions used. Otherwise the sample solution to be examined interferes with the test.
If the preparation under test does not comply with the test at a dilution less than the MVD, repeat the test using a greater dilution, not exceeding the MVD. The use of a more sensitive lysate permits a greater dilution of the preparation to be examined and this may contribute to the elimination of interference.
Interference may be overcome by suitable treatment, such as filtration, neutralization, dialysis or heat treatment. To establish that the treatment chosen effectively eliminates interferencewithout loss of endotoxins, perform the assay described above using the preparation to be examined to which standard endotoxin has been added and which has then been submitted to the chosen treatment.
Prepare the solutions A, B, C and D according to Table 2, and perform the test on these solutions following the procedure in Test for confirmation of labeled lysate sensitivity under Preparatory testing.
Note:- Prepare the solution A and the positive product control solution B using a dilution not greater than the MVD and treatments as for the Test for interfering factors under Preparatory testing. The positive control solutions B and C contain the standard endotoxin preparation at a concentration corresponding to twice the labeled lysate sensitivity. The negative control solution D consists of water BET.
Interpretation:- The test is considered valid when both replicates of solution B and C are positive and those of solution D are negative.
When a negative result is found for both replicates of solution A, the preparation under test complies with the test.
When a positive result is found for both replicates of solution A, the preparation under test does not comply with the test.
When a positive result is found for one replicate of solution A and a negative result is found for the other, repeat the test. In the repeat test, the preparation under test complies with the test if a negative result is found for both replicates of solution A. The preparation does not comply with the test if a positive result is found for one or both replicates of solution A.
However, if the preparation does not comply with the test at a dilution less than the MVD, the test may be repeated using a greater dilution, not exceeding the MVD.
The test quantifies bacterial endotoxins in sample solutions by titration to an endpoint. Prepare the solutions A, B, C and D following Table 3, and test these solutions according to the procedure in Test for confirmation of labeled lysate sensitivity under Preparatory testing.
Solution A: Sample solution under test at the dilution, not to exceed the MVD, with which the test for interfering factors was completed. Subsequent dilution of the sample solution must not exceed the MVD. Use water BET to make a dilution series of four tubes containing the sample solution under test at concentrations of 1, 1/2, 1/4 and 1/8 relative to the concentration used in the test for interfering factors. Other dilutions up to the MVD may be used as appropriate.
– Solution B: Solution A containing standard endotoxin at a concentration of 2 λ (positive product control)
– Solution C: A dilution series of four tubes of water BET containing the standard endotoxin at a concentration of 2λ, λ, 0.5λ and 0.25λ, respectively
– Solution D: Water BET (negative control)
Calculation and interpretation
The test is considered valid when the following three conditions are met.
1: Both replicates of the negative control solution D are negative.
2: Both replicates of the positive product control solution B are positive.
3: The geometric mean endpoint concentration of the solution C is in the range of 0.5λ to 2λ.
To determine the endotoxin concentration of solution A, calculate the endpoint concentration for each replicate by multiplying each endpoint dilution factor by λ.
The endotoxin concentration in the sample solution is the endpoint concentration of the replicates. If the test is conducted with a diluted sample solution, calculate the concentration of endotoxin in the original sample solution by multiplying by the dilution factor.
If none of the dilutions of sample solution is positive in a valid assay, report the endotoxin concentration as less than λ (if diluted sample was tested, report as less than the smallest dilution factor of the sample multiplied by λ). If all dilutions are positive, the endotoxin concentration is reported as equal to or greater than the largest dilution factor multiplied by λ (e.g. initial dilution factor multiplied by 8 and by λ in Table 3).
The preparation under test meets the requirements of the test if the concentration of endotoxin in both replicates is less than that specified in the individual monograph.
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