Biomedical Engineering Reference
In-Depth Information
ATH that was signifi cantly longer than that for UFH and persisted up to 3 h after stoppage of perfu-
sion.
250
The prolonged intravenous pharmacokinetics of Mitra and Jordan ATH over short half-life
of UFH
273
would allow lower dosages to give successful prophylactic effects. Actual antithrombotic
comparisons between the Mitra and Jordan product and other heparins
in vivo
may be warranted.
Extensive
in vivo
investigation of the Chan et al. keto-amine-linked ATH has been accom-
plished. Half-life studies in rabbits disclosed an intravenous α-phase (two-compartment model)
half-life for both mass assay and anti-FXa activity of 2.5-2.6 h (Table 18.2), eightfold longer than
UFH and fi vefold shorter than the α-phase of AT.
141
The β-phase (three-compartment model) intra-
venous half-life for keto-amine ATH was measured to be 13 h (Table 18.2),
141
which was the lon-
gest disappearance value reported for ATH complexes to date. Investigation of nonselective plasma
protein binding, a known mechanism for elimination of heparin activity, showed that Chan et al.
complex had less binding than UFH, approaching that of LMWH.
274
Additional experiments indi-
cated that uptake onto endothelial surfaces remarkably decreased for the keto-amine conjugate
compared to UFH in noncovalent AT-UFH complexes,
274
which would allay loss of this ATH from
the circulation by vessel wall binding. Given that Chan et al. ATH is prepared by simple incubation
of unaltered commercial UFH with AT, an intriguing possibility exists that such a stable complex
might already be generated from aldose-heparinoids
in vivo
. Attempts to isolate ATH after several
bolus UFH injections were successful in obtaining a small quantity of identifi able product,
261
con-
sistent with the potential for generation of conjugate from vascular AT and endogenous, nonprotein
linked, heparin chains that are known to circulate in humans.
275-277
Subcutaneous administration
of Chan et al. ATH resulted in very poor systemic adsorption but activities persisted up to 96 h
after injection, with peak levels occurring at 24-30 h.
141
The much reduced capability, relative to
UFH,
40
of the large keto-amine conjugate to pass through extravascular tissue suggested the ATH
may be retained in sequestered spaces for applications such as anticoagulation treatment in the
lung during RDS.
214,278-281
As a confi rmation, intratracheal instillation of Chan et al. ATH into
adult rabbits resulted in signifi cant AT concentrations and anti-FXa activity in lavage fl uids 48 h
after treatment, without any of the human ATH antigen detected systemically.
141
Further experi-
ments in full-term newborn rats demonstrated detectable anti-FXa activity in lavage fl uid even 96
h after airway instillation of the complex. Moreover, even if lavaging did not retrieve the entire
instilled activity and there was no evidence of ATH permeability into the circulation, anticoagulant
or antithrombotic functioning of ATH on internal lung surfaces would still be the primary pur-
pose for sequestration as premature newborn lungs develop. As evidence, experiments showed that
Chan et al. ATH inhibited plasma-thrombin generation on fetal-distal-lung epithelium
in vitro
much
more effectively than noncovalent AT and UFH,
282
or even covalent complexes of UFH or derma-
tan sulfate with heparin cofactor II
283
prepared by similar Schiff base or Amadori rearrangement
chemistry.
284
Analysis of the data showed that the rapid rate of direct, noncatalytic inhibition of
initial thrombin feedback activation of the cascade was a critical part of ATH's enhanced ability to
inhibit expression of thrombin activity on epithelial surfaces.
282
Animal studies also validated keto-
amine ATH as a superior systemic antithrombotic agent. Preformed thrombi in a rabbit jugular vein
model had lower fi brin accretion and were reduced in size by a single ATH bolus injection, while
clot size increased with similar mg kg
-
1
doses of AT
UFH noncovalent mixtures.
285
Other experi-
ments showed that, at equivalent anti-FXa doses, Chan et al. ATH possessed signifi cantly greater
prophylactic capacity against thrombi formation than UFH in a rabbit arterial thrombosis preven-
tion model.
286
Examination of interactions with fi brin-monomer proved that the covalent complex
could prevent clot formation and extension by neutralizing fi brin-bound thrombin,
286,287
which can
become protected from noncovalent AT-UFH by inclusion within thrombin-UFH-fi brin ternary
complexes.
288
Hemorrhagic side effects were reduced for ATH versus infused equimolar mixtures
of AT
+
UFH at similar plasma anti-FXa activities in rabbit bleeding ear experiments.
285
Indeed,
bleeding risk to antithrombotic benefi t analysis demonstrated a highly signifi cant improvement over
UFH for the ATH formed by Schiff base or Amadori rearrangement.
286
+
