Biomedical Engineering Reference
In-Depth Information
ATH prepared by Chan et al. displayed a rate for direct neutralization of FXa (Table 18.1) that
was on par with the maximal value for noncovalent AT
high-affi nity heparin. 264 With respect to
thrombin, ATH prepared by Chan et al. possessed an extremely rapid second-order rate constant of
3.1
+
10 9 M - 1 s - 1 (Table 18.1), 141 one of the fastest ever reported. This potency may well have been
derived from the fact that, unlike other ATH products, unmodifi ed (undamaged) UFH is used in
high molar excess to prepare the Chan et al. complex, which allows for selection by AT of the most
active subpopulation of GAGs. Chan et al. ATH is also augmented during synthesis by the preferen-
tial coupling of heparin to the β-isoform of AT which, in combination with heparin, reacts at a two-
fold faster rate than that for α-AT. 265 Further studies have established that Chan et al. ATH directly
inhibits many of the activated coagulation factors at a much faster rate than noncovalent AT
×
+
UFH mixtures. 266 An unexpected fi nding was that the Chan et al. conjugate had potent activity for
catalysis of exogenous AT inhibition of FXa and thrombin that was greater than the starting UFH or
high-AT-affi nity heparin (Table 18.1). 141 From in-depth studies, it has been discovered that heparin
chains of ATH have at least one pentasaccharide and some 30-50% of the ATH heparin moieties
have at least two high-AT-affi nity sequences. 264 Other workers have reported that 1-3% of UFH
molecules indeed have more than one high-affi nity binding site for AT. 142,267 Given the vast surfeit
of UFH over AT during reaction, the long incubation times during synthesis allowed for sampling
of heparin molecules by AT until binding occurs by a pentasaccharide that is close to the aldose for
covalent linkage to occur. In this sense, multipentasaccharide heparins would be preferred since
dissociation by AT from one pentasaccharide would more likely lead to a short diffusion to the
second pentasaccharide sequence on the same heparin chain (closer to the aldose terminus), as
opposed to a longer distance of travel between heparin molecules. Further in vitro investigations
have confi rmed the anticoagulant utility of Chan et al. ATH. Comparisons between the conjugate
and various heparinoids in activated plasma systems showed Chan et al. ATH to have superior effi -
cacy in controlling thrombin generation. The covalent complex was more effective than either UFH
or LMWH, at equivalent anti-FXa doses, in preventing contact phase-activated thrombin generation
in adults, children, or newborn plasmas. 268 Experiments with a variety of activators further illus-
trated the increased avidity of Chan et al.'s ATH for inhibiting thrombin generation in hemophilic
plasma relative to UFH and LMWH. 269 In all cases, the rapid neutralization by the Schiff base or
Amadori ATH complex of the initial thrombin responsible for feedback activation of the cascade,
coupled with the strong catalytic capacity to prevent further propagation of thrombin generation,
would enhance its capability over other heparinoids.
18.5.3 E FFECTS In Vivo
In vivo testing of ATH complexes has been reported, with much of the data describing recent work
with the Chan et al. compound. Primary investigations give information on pharmacokinetics,
while some antithrombotic and hemorrhagic results are available from pathological animal models
to assess functional potential in clinical disease states. A partial collection of pharmacokinetic data
is given in Table 18.2.
ATH produced by Ceustermans et al. from N-desulfated, high-affi nity heparin displayed bipha-
sic disappearance from plasma in rabbits consisting of a short fi rst phase or α-phase and slightly
longer second or β-phase (Table 18.2). 231 In comparison, Ceustermans et al. ATH made from high-
affi nity hexyl-amino-modifi ed heparin had a single α-phase elimination from plasma of slightly lon-
ger duration (Table 18.2). 231 Plasma half-lives for both these compounds were not more than three
times longer than that for UFH in rabbits (single α-phase of 0.23
±
0.03 h) and much shorter than
0.4 h determined for AT in the rabbit model. 231 Hastening of elimination for the hexyl-
amino ATH complexes could well be expected through interaction with plasma and cell surface
proteins, given the reports of binding by these ATH materials to histidine-rich glycoprotein. 172
ATH preparations of hexyl-amino derivatized LMWHs gave improved intravenous β-phase half-
lives of 7-8 h (Table 18.2), 270,271 which was again about threefold longer than the corresponding
the 11.0
±
 
Search WWH ::




Custom Search