Expert view: Improving the endotoxin test paradigm established by LAL
THE ADVANTAGES of the recombinant Factor C (rFC) test lie in its (a) scientific characterisation, (b) sustainability and (c) specificity. In light of diminishing horseshoe crab populations, sustainability has been mandated via regulations that include the concept of the “3 Rs”: replacement, reduction and refinement of analytical tests and reagents that use or are derived from animals.
The rFC test possesses several distinct advantages over the LAL assay and are here referred to as the “3 S’s”.
The lack of characterisation of LAL has never been an acknowledged problem. However, LAL was used for a decade before an alternative activation mechanism was discovered (the Factor G pathway). It was a shocking revelation at the time that a common microbial substance other than endotoxin could activate the Limulus cascade to produce a positive reaction when little or no endotoxin was present.
rFC contrasts with LAL as it consists of a single recombinant protein and a small fluorescent peptide and thus no Factor G. The protein content of rFC can be measured to perfection, whereas the proteins present in LAL include at least eight: (i) Factor C, (ii) Factor B, (iii) proclotting enzyme, (iv) coagulogen, (v) Factor G and (vi-viii) serine protease inhibitors 1, 2 and 3. In the LAL milieu, some of these proteins are purposefully denatured by processing (inhibitors) and likely other proteins exist that are not specified.