© 1995 American Heart Association, Inc.
Articles |
Inhibitors for the In Vitro Assembly of Lp(a)
From the Medical Biochemistry Department, University of Graz, and the Second Department of Medicine, University Hospital of Vienna (Austria) (K.K.).
Correspondence to Prof Dr G.M. Kostner, Medical Biochemistry Department, Karl-Franzens-Universität, Harrachgasse 21/III, 8010 Graz, Austria.
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Abstract |
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Abstract Lp(a) is composed of an LDL-like core and the glycoprotein apo(a). Current evidence strongly suggests that the assembly of this atherogenic lipoprotein proceeds outside the liver cells in a two-step fashion. In the first step, a loose complex is formed involving kringle-4 motifs in apo(a) and one or more Lys side chains in apoB-100. In the second step, this complex is stabilized by a disulfide bridge. Indications are that Lp(a) assembly is critical in the determination of plasma apo(a) concentrations. Therefore, we searched for substances that interfere with the first step of Lp(a) assembly.
-Aminohexoic acid (-AHA), known as an
inhibitor from earlier assembly studies, had an
IC50 of 4.8 mmol/L. The IC50 of Pro,
HO-p-aminobenzene sulfonamide, Lys,
N--acetyl-Lys, taurine, Glu, serotonin,
and benzamidine were all >20 mmol/L. 
-Aminobutyric acid,
spermine, and spermidine exhibited IC50 on the same
order of magnitude as -AHA. The substances with the highest
inhibitory action were tranexamic acid and
-aminovaleric acid. Seven of eight patients treated in a pilot
study with tranexamic acid (Cyclocapron) responded with a decrease of
plasma apo(a) of 18.5±8.2%. We suggest that substances that interfere
with the Lp(a) assembly are worth pursuing further for their usefulness
as therapeutic agents in reducing high plasma Lp(a) concentrations.
Key Words: tranexamic acid • fibrinolysis • drug treatment • atherosclerosis • recombinant apo(a)
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Introduction |
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TopAbstractIntroductionMethodsResultsDiscussionReferences |
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Evidence is accumulating that Lp(a) is a severe proatherogenic and prothrombotic serum lipoprotein.1 2 3 4 5 6 Although the conclusions are not entirely consistent,7 most of the previous studies identified Lp(a) as a risk factor for myocardial infarction8 and stroke.9 Lp(a) has been postulated as a strong independent marker for premature coronary heart disease.10 11 In a recent report of 118 men undergoing coronary arteriography, Lp(a) was the only lipid parameter in a multivariate analysis that showed a significant correlation with vessel score (number of affected vessels), stenosis score (severity of stenoses), and extent score (length extension of lesions).12
Lp(a) is composed of an LDL-like core and apo(a). Apo(a) resembles
plasminogen in structure and its affinity to
Lys-containing proteins. Apo(a) is a glycoprotein with
a protease domain, a single kringle-5 motif, and several kringle-4
domains, all of which are homologous to corresponding structures in
plasminogen.13 All apo(a) isoforms contain one
set each of kringle-4 subtypes (T1 and T3 through T10) and a
variable number of kringle-4 T2 domains (for kringle nomenclature,
see Fig 1
and References 13 and 1413 14 ). More than 40 potential
genetic isoforms of apo(a) exist because of variations in the number of
kringle-4 T2 repeats.15
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The metabolic steps involved in the biosynthesis of Lp(a) are just beginning to be unraveled. Earlier studies by our group demonstrated that VLDL is not a direct precursor of Lp(a).16 Several research groups, including our own, also showed that individuals with high plasma levels have a high rate of Lp(a) synthesis.17 18 However, no correlation with the fractional catabolic rate of Lp(a) was found.19 The consecutive steps involved in apo(a) biosynthesis and secretion, including the assembly of apoB-containing lipoproteins to form the Lp(a) lipoprotein in its native form, are the subjects of several recent investigations. Azrolan et al20 demonstrated that in the cynomolgus monkey plasma Lp(a) concentrations correlated inversely with apo(a) size but directly with hepatic apo(a) mRNA abundance. White et al21 examined the molecular basis for the inverse correlation between apo(a) size and plasma Lp(a) concentration using primary cultures of baboon hepatocytes. They found that the residence time in the endoplasmatic reticulum of secreted apo(a) correlates with apo(a) size. Also, apo(a) "null" alleles were transcribed and gave rise to a protein that, however, was intracellularly degraded and therefore not secreted.
Several research groups addressed the question of whether Lp(a)
assembly occurs intracellularly or extracellularly. Two forms of apo(a)
differing in molecular weight are synthesized by baboon
hepatocytes in culture; only the larger variant is secreted
into the incubation medium. Using specific antibodies, White et
al22 showed that apo(a) coprecipitates with apoB only from
culture medium, not from cell lysates. Through the use of ELISA and
specific antibodies against apo(a) and apoB, a similar study of nine
human liver biopsies was unable to detect an association of apo(a) and
apoB in cell lysates, suggesting that apo(a) is not coupled to apoB
within the liver cells and that Lp(a) assembly occurs outside the
cell23 or in part at the cell surface.24
Other groups studied the structural features of apo(a) necessary for
Lp(a) assembly. They found that the unique free Cys in kringle-4 T9 of
apo(a) forms a disulfide bridge with Cys 3734 in
apoB-100.25 26 27 -AHA prevented the formation of an
apo(a):apoB-100 complex,27 28 suggesting that the assembly
proceeds in two steps: first, apo(a) associates with LDL mediated by
the interaction of kringle-4 with a Lys group in apoB-100; second, the
-S-S- linkage forms. Whether a specific enzyme is involved in the
second step of assembly is still being disputed.
Considering all these studies together, we may be tempted to assume that any substance interfering with the assembly of Lp(a) may have a measurable effect on plasma Lp(a) levels. This has, in fact, been postulated for N-acetyl Cys, a substance that prevents the oxidation of cysteine to cystine29 and thus interferes with the second step of complex formation. The aim of this study was to search for a group of substances that prevent the first step of Lp(a) assembly. If such substances prove effective in vivo, their usefulness as therapeutic agents for reducing plasma Lp(a) levels in patients at high risk for vascular diseases could be tested. This type of research is being pursued in our laboratory.
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Methods |
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Materials
Antibodies specific for apo(a) and apoB were prepared in our own laboratory.6 8 Lp(a) reference sera and isoform standards were obtained from Immuno AG. Horseradish peroxidase–labeled protein A and an ECL kit were purchased from Amersham Corp. Replication-defective biotinylated adenovirus (dl312), streptavidin-polylysine conjugate, and human polylysine-transferrin conjugate were prepared at the Institute of Molecular Pathology (Vienna, Austria). DMEM and FCS were obtained from Böhringer Mannheim Corp. COS-7 cells were from American Type Tissue Culture (CRL-1651). Nitrocellulose was from Hoefer Scientific. All other chemicals were from E. Merck or Sigma Chemical Co.
Isolation of LDL
LDL in a density range of 1.023 to 1.060 was prepared from
pooled serum of fasting healthy subjects with low Lp(a) concentrations
(<5 mg/100 mL) by preparative ultracentrifugation
at stepwise increasing densities as described
earlier.16 17 To avoid LDL oxidation, all procedures were
performed under nitrogen and in the presence of 0.5 g/L
Na2EDTA. LDL was subjected to immunoadsorption for removal
of contaminating Lp(a).27 The final product was >97%
pure, as verified by chemical and immunochemical analyses. It
was stored under nitrogen for a maximum of 3 to 5 days at 4°C before
use.
Construction of Apo(a) Expression Vectors and
Transferrinfection
The expression plasmids for apo(a) containing sequences coding
for 18 kringle-4, kringle-5, and the protease domains were assembled
from cDNA clones reported by McLean et al30 as described
in detail earlier.14 27 Apo(a) cDNA fragments were ligated
into the EcoRI site of pSG 5. Plasmid DNA was isolated by
the alkaline lysis method and purified by CsCl density gradient
ultracentrifugation. Plasmids coding for two
different apo(a) sequences were assembled (Fig 1
). The WT 7.4-kB
construct contained the signal peptide, all nine unique kringle-4s,
nine repetitive kringle-4 T2's, kringle-5, and the protease domain.
The mutant construct was identical to the WT except that the unique
free cysteine (Cys 4057) in kringle-4 T9 was replaced by Arg by use of
site-directed mutagenesis.27 To obtain highly
efficient expression of apo(a) recombinants, thereby allowing us to use
immunochemical detection of apo(a) in subsequent experiments and thus
avoid radiolabeled pulse-chase experiments, COS-7 cells were
transfected by a receptor-mediated gene delivery system
(transferrinfection) as outlined in detail
previously.14 27 Cells were cultivated in 10% FCS at
approximately 300 000 cells per P60 dish and incubated for 4 hours
with the expression vector conjugated to streptavidin-polylysine
and the biotinylated adenovirus. The medium was then replaced by
DMEM–20% delipidated FCS, and the cells were incubated for 24 to 48
hours. The medium was harvested; mixed with the protease
inhibitors aprotinin (0.2 µmol/L), leupeptin (50
µmol/L), and PMSF (1 µmol/L); and frozen in 1-mL aliquots. The
content of recombinant apo(a) ranged between 500 and 800 ng/mL.
Association of Apo(a) With LDL
We previously showed that on incubation of straight transfection
medium containing recombinant apo(a) and LDL at ratios between 1:2 and
1:10 for 18 to 24 hours at 37°C, up to 90% of apo(a) is complexed to
LDL, even in the absence of cells. The resulting "artificial"
Lp(a) resembles native Lp(a) in flotation behavior in density gradient
ultracentrifugation, electrophoretic migration, and
chemical and immunochemical properties. For the present study, we
standardized the assembly assay as follows. The cell medium was
adjusted to an apo(a) concentration of 500 ng/mL and a final LDL
concentration of 5 mg/mL protein and incubated for 18 hours at 37°C.
To study the effect of inhibitors on the Lp(a) assembly,
increasing amounts of -AHA, homologues of -AHA, or other
substances were added to the medium before the LDL was added. Because
50 mmol/L -AHA is able to dissociate apo(a):LDL complexes that are
not stabilized by -S-S- bonds,14 this agent was added when
appropriate after the medium was incubated with LDL and just before
immunochemical quantification of the amount of stable apo(a):LDL
complexes.
Monitoring the Efficiency of the Assembly
SDS–agarose gel electrophoresis was performed with 1.5% gels
containing 0.1% SDS±1% mercaptoethanol followed by transblotting to
nitrocellulose (overnight at 4°C), incubation with specific
antibodies against apo(a) or apoB, and visualization of the bands by
the ECL method.14 27 As a reference, the isoform standard
from Immuno AG containing five apo(a) isoforms with 14, 19, 22, 27 and
35 kringle-4 repeats was used. This method provides only qualitative
results.
Quantitative analysis was attained by DELFIA with two sets of
antibody combinations, anti-apo(a) and anti-apoB. The
immunoquantification of free apo(a) and apo(a):LDL complexes by DELFIA
was performed as described previously.14 27 Briefly,
96-well Costar plates were coated with affinity-purified,
polyclonal "capture" antibodies from sheep against apo(a) or
apoB. These antibodies were free of cross-reactivity with other
proteins. Nonspecific binding sites on the plates were blocked with 250
mL of 4% skim milk powder in 50 mmol/L Tris:HCl (pH 7.7, assay
buffer). After extensive washing, 200-mL aliquots of samples were added
to the wells and incubated for 2 hours at room temperature. Plates were
washed three times with the assay buffer, followed by the addition of
200 mL detection antibody, europium-labeled anti-apo(a) or
anti-apoB from rabbit, and further incubated for 2 hours at room
temperature. After three washing steps, 200 mL enhancement solution
(Pharmacia) was added and the fluorescence was measured in a
DELFIA reader (Pharmacia). By using different combinations of capture
and detection antibodies, we could determine total apo(a), total apoB,
and apo(a):apoB complexes. Standard curves were produced with a Lp(a)
reference standard (Immuno AG). The assay was linear from 1 to 100 ng
of apo(a) or apoB per well; the coefficient of variation was 2.6%. In
control experiments, possible influences of the substances listed in
Table 2 on the DELFIA were tested. Concentrations of these substances
to 50 mmol/L produced no significant effects on the assay.
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Other Assays
Cholesterol and triglycerides were
measured enzymatically with reagents from Böhringer Mannheim
Corp. Protein was measured according to Lowry's method.
Influence of Antifibrinolytic Therapy on Plasma Lp(a)
Levels
In a pilot study involving eight patients treated with
tranexamic acid (Cyclocapron) for antifibrinolytic therapy, we followed
the course of plasma Lp(a) concentrations over a period of 5 to 6 days.
One woman (M.M.) and one man (M.F.), 60 and 44 years of age,
respectively, were treated with 500 mg Cyclocapron IV once a day. A
third man (N.H.) received 500 mg Cyclocapron IV three times a day. Five
other men (Z.A., T.D., K.R., K.G., and F.S.), between 42 and 72 years
of age, received an oral dose of 500 mg Cyclocapron three times a day.
Blood was drawn in the morning after a 12-hour fast, and Lp(a) was
assayed by DELFIA. None of the patients were on lipid-lowering or
hormone therapy at least 2 weeks before the start of and during the
study.
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Results |
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Incubation of cell medium containing 500 ng/mL recombinant apo(a) with 5 mg of LDL protein for 18 hours at 37°C leads to the formation of LDL:apo(a) complexes. It was shown previously that these complexes resemble native Lp(a) chemically and physicochemically.27 Fig 2
shows a Western blot that demonstrates that the
association products between WT recombinant apo(a) and apoB-100
formed during the 18-hour incubation remain stable in 0.1% SDS or 50
mmol/L -AHA. When 50 mmol/L -AHA was added to the medium before
the incubation with LDL, no apo(a):apoB-100 complex was seen (Fig 2,
lane 3). The Cys mutant of recombinant apo(a) did not result in any
visible complex with apoB when this assay system was used (Fig 2, lane
5). SDS-PAGE of media from control experiments in which cultures were
incubated without LDL revealed only one band corresponding to lane 3 or
5 in Fig 2 (data not shown).
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To study the assembly quantitatively, experiments similar to that shown
in Fig 2 were performed in which the efficiency of the assembly was
monitored by DELFIA (Table 1). Under the chosen
conditions, 67.3% of WT recombinant apo(a) produced stable complexes
with LDL. Addition of 50 mmol/L -AHA before incubation with LDL
reduced the complex formation to 2.6%. -AHA, however, had little
effect when added to the mixture after an 18-hour incubation of LDL
with recombinant apo(a). On the other hand, 1% mercaptoethanol reduced
the complex formation to <2%, whether added before or after
incubation with LDL. Control experiments confirmed that under the
chosen conditions mercaptoethanol did not interfere with our DELFIA
assay. The Cys mutant of recombinant apo(a) in which Arg was
substituted for the free Cys group (amino acid 4057) exhibited only
18.3% assembly with LDL in the absence of inhibitors; the
assembly, however, was completely abolished by -AHA and
mercaptoethanol, regardless of whether the substances were added before
or after incubation with LDL.
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From earlier work, we knew that recombinant apo(a) assembly with LDL is
not affected by NaCl (up to 10%) and that nonionic and anionic
detergents interfere with the assembly.27 Taking all these
data together, we concluded that the stable assembly of Lp(a) proceeds
in two steps. First, one or several kringle-4s of apo(a) bind to Lys
side chains of LDL. This association is prevented by -AHA or 0.1%
SDS but is insensitive to high salt concentrations. Second, an -S-S-
bridge is formed between WT apo(a) and apoB that is sensitive to
reducing agents such as mercaptoethanol. Mutant recombinant apo(a)
reacts only noncovalently with apoB yet is unable to form stable
complexes.
In subsequent experiments, a series of compounds was tested that we
hypothesized would interfere with the first step of Lp(a) assembly.
Increasing amounts of these substances were added to the medium
containing 500 ng of WT recombinant apo(a); then 5 mg LDL was added.
The amount of apo(a):LDL complexes was quantified by DELFIA and
expressed in relation to the Lp(a) assembly in the absence of
inhibitors. The concentration of the inhibitory
substances was plotted against the relative amount of complexes (in
percent) in a log-linear scale. From this plot, the
IC50 was extrapolated. Fig 3 shows a
characteristic plot obtained with tranexamic acid, -AHA, and
N--acetyl Lys. Table 2 lists the
IC50 values of all studied compounds with measurable
inhibitory capacity.
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Lys alone turned out to be a rather weak inhibitor,
with an IC50 of 38 mmol/L. Acetylation of the
-amino group increased the IC50 to 52 mmol/L. On the
other hand, 
-acetylation of Lys reduced
IC50 to <10 mmol/L. The other substances were selected
partly on the basis of their known interference of
plasminogen binding to fibrin or their structural homology
to these substances. The listed compounds can be grouped into three
categories. The first group contains substances with
IC50<35>10 mmol/L: p-aminomethylbenzene
sulfonamide, HO-Pro, and Pro. The second category comprises substances
with IC50<10>1: N--acetyl-Lys,
ornithine, spermidine, spermine, -AHA, and -aminobutyric
acid. The third category contains those substances with
IC50<1 mmol/L: -aminovaleric acid and tranexamic
acid.
In Vivo Effect of Tranexamic Acid on Plasma Apo(a)
Levels
Tranexamic acid, sold as Cyclocapron by KabiVitrum Stockholm, is
recommended as an antifibrinolytic agent. According to the
manufacturer, one oral dose of 20 mg/kg body wt yields plasma
concentrations of 0.08 to 0.16 mmol/L. In a pilot study, we followed
the short-term effect of Cyclocapron on apo(a) plasma
concentrations in eight patients with various apo(a) phenotypes
who underwent an antifibrinolytic therapy (Table 3).
Three patients received an intravenous infusion of
Cyclocapron. Two responded with a >20% reduction in apo(a). In one
patient (M.F.), Cyclocapron was not effective. Five other patients were
treated with an oral dose of 1.5 g three times a day (morning, noon,
and evening). Table 3 shows the course of apo(a) plasma concentrations.
The five patients reacted with a mean decrease of apo(a) ranging from
11% to 26%. The observed efficacy of Cyclocapron was not related to
the apo(a) phenotype. We also measured Lp(a) values in some
patients by an apo(a):apoB DELFIA and found comparable results (data
not shown).
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Discussion |
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TopAbstractIntroductionMethodsResultsDiscussionReferences |
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Many attempts have been made to interfere with increased plasma Lp(a) levels by diet or medication.31 Except for a few reports on foods rich in saturated fat,32 diets are believed to be of little use in reducing Lp(a) concentrations. On the other hand, it is known that alcohol intake significantly reduces Lp(a).33 34 With respect to lipid-lowering drugs, nicotinic acid and its derivatives appear to have a moderate lowering effect on Lp(a).35 36 Anabolic sterols were shown to have a pronounced effect, reducing Lp(a) levels by up to 78.6%.37 38 Lp(a) binds weakly to the LDL receptor in vitro,39 and LDL receptors probably play a minor role in the in vivo catabolism of Lp(a).40 This is probably the reason why HMG–coenzyme A reductase inhibitors, the most potent drugs currently on the market for reducing LDL, are of no use in reducing Lp(a).41 The strategy for developing Lp(a)-lowering drugs is therefore centered on the interference of the rate of Lp(a) biosynthesis, which determines plasma Lp(a) levels.17 18 Because factors that regulate apo(a) transcription are currently poorly understood, much effort is directed to posttranscriptional events related primarily to the assembly of apo(a) with LDL. This was the rationale behind the suggestion of using N-acetyl cysteine as an Lp(a) reducing agent29 because N-acetyl cysteine blocks the formation of the disulfide bond between apo(a) and apoB in vitro. However, this drug reduced Lp(a) levels by only 7% in individuals with high plasma concentrations (42 to 202 mg/dL).42 Because the assembly of Lp(a) proceeds in two steps,22 23 24 25 26 27 28 it was the aim of this study to systematically investigate substances that interfere with the first step of the assembly.
As pointed out above, Lys residues on apoB are bound to kringle-4 motifs in apo(a), followed by the stabilization of this loose complex by a disulfide bridge.22 23 24 25 26 27 28 Although the picture is not yet complete, we recently obtained additional information on the structural requirements of LDL for the Lp(a) assembly: four homozygous individuals suffering from lecithin-cholesterol acetyltransferse (LCAT) deficiency completely lacked apo(a) in their plasma.43 The LDL fraction isolated from plasma failed to assemble with recombinant apo(a) in an experimental design similar to that in this study. We also studied LDL in a patient with homozygous familial defective hyperapobetalipoproteinemia (FDB), which is known to possess an altered structure and thus binds only weakly to the LDL receptor. The capability to assemble the FDB–apoB: recombinant apo(a) complex was reduced to approximately 50%.44 From these data, we conclude that a defined surface structure of apoB in LDL is one prerequisite for forming intact Lp(a). In analogy to the well-known mechanism of the interaction of kringle motifs in plasminogen with Lys groups in fibrin, it is assumed that kringle-4 is necessary for the first step of complexing apo(a) to apoB. To pinpoint the role of the different kringle-4 types of apo(a) in the assembly, a variety of different constructs of recombinant apo(a) have been produced with defined sets of unique and repetitive kringle-4s.14 From this study, we postulate that (1) the unique kringle-4 T6 is primarily responsible for Lys binding in apoB, (2) all the other kringle-4s play no major role in the first step of Lp(a) assembly, (3) kringle-4 T9 is necessary for the second step of assembly, and (4) the distance between kringle-4 T6 and T-9 is critical for efficient Lp(a) assembly.
From the absence of Lp(a) in LCAT deficiency43 and the
report of Hobbs et al45 that transgenic mice with the
human apo(a) gene have significantly lower plasma apo(a) levels
compared with transgenic mice containing the human apo(a) plus apoB-100
gene, we hypothesize that free apo(a) not complexed to LDL is removed
faster from plasma and may yield a reduction in the plasma apo(a)
concentration. With this concept in mind, we searched for substances
that interfere with the Lp(a) assembly. Earlier investigations already
showed that -AHA prevents the formation of Lp(a) from LDL and
recombinant apo(a).14 25 28 Here we report on the potency
of various substances with structural homology to -AHA with respect
to their interference in the Lp(a) assembly.
Lys by itself was able to interfere only at relatively high
concentrations (IC50, 38 mmol/L). The
acetylation of the -amino group increased
IC50 to 52 mmol/L, and the acetylation of the
-amino group led to a reduction by a factor of 4. In numerous
other tests of amino acids, only HO-Pro and Pro had measurable
inhibitory activity in a decreasing order of magnitude.
With this in mind, it is noteworthy that Trieu et al46
reported earlier that recombinant apo(a) binding to
LDL2 is competitively inhibited by Pro and HO-Pro.
From this observation, it was suggested that apo(a) may exert its real
physiological role by binding to
subendothelial matrix, ie, domains rich in Pro and
HO-Pro such as collagen and elastin. Here we describe several compounds
with significantly higher inhibitory action than Pro or
HO-Pro: N--acetyl-Lys, ornithine, spermidine,
spermine, and -aminobutyric acid exhibited an inhibition
comparable to that of -AHA. It is of interest that several
physiologically active substances can be found
in this list. It will be interesting to see whether the
physiological role of Lp(a) might be linked to any
of these compounds.
Benzamidine, which is known to bind specifically to kringle-5 in
plasminogen, was without effect in our Lp(a) assembly
assay. We also found substances with an ID50 of <1.0:
tranexamic acid, a substance used in humans for antifibrinolytic
therapy (IC50, 0.65 mmol/L), and
-aminovaleric acid (IC50, 0.76 mmol/L).
Tranexamic acid was more than 7 times more effective than -AHA. It
is noteworthy that the antifibrinolytic action of tranexamic acid is 10
times higher than that of -AHA.47
We should mention here that control experiments with purified
recombinant apo(a) obtained from Genentech instead of recombinant
apo(a)–rich medium were also performed as pointed out in Reference 4444 .
The IC50 values using -AHA or tranexamic acid were
comparable to those listed in Table 2, demonstrating that the system we
were using for the other experiments was adequate.
Whether or not one or the other substance listed in Table 2 might be of
relevance for human use in suppressing increased plasma Lp(a) levels
cannot be ascertained on the basis of this study and awaits further in
vivo experiments. The relevance for human use will probably depend on
the ability to dissect the antifibrinolytic action from the potency to
interfere with the Lp(a) assembly. In addition, for such potential
drugs to be active, they should not be sequestered by high plasma
concentrations of plasminogen. It might be also worth
noting that -AHA exerts its antifibrinolytic action by blocking the
Lys binding sites in plasmin, thereby preventing the binding to
fibrinogen; on the other hand, -AHA has also been shown to interfere
with the inactivation of plasmin by 2-macroglobulin, a
process that greatly prolongs the proteolytic activity of plasmin in
plasma.48
In a short-term pilot study in eight patients, two who had highly
elevated plasma Lp(a) levels, it was reassuring to note that seven
patients responded to tranexamic acid therapy with a measurable
reduction in apo(a). The applied oral dose of Cyclocapron 1500 mg three
times a day led to a tranexamic acid plasma concentration on the order
of 0.3 mmol/L.49 In comparison, a single 500-mg dose of
Cyclocapron given intravenously yields a calculated initial
plasma concentration of >1 mmol/L. Considering a half-life of 80
hours and an IC50 of 0.65 mmol/L (Table 2) for
Cyclocapron,48 one may well envisage a therapeutic effect
at the given doses. We must stress, however, that this was a pilot
experiment and that the patients involved were not selected for this
study but were treated with Cyclocapron as antifibrinolytic therapy. We
are also tempted to assume that additional compounds that have not been
tested here might exist with an agent that reduces Lp(a) assembly as
well as or better than Cyclocapron but with a low antifibrinolytic
activity. Studies along these lines are currently being pursued in our
laboratory.
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Selected Abbreviations and Acronyms |
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Acknowledgments |
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This work was supported by the Austrian Research Foundation, grants S-7104 and SFB F007. The technical assistance of A. Ibovnik, M. Stultschnig, and H. Grillhofer is appreciated.
Received May 22, 1995; accepted August 7, 1995.
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