CLINICAL PHARMACOLOGY
Alpha1-antitrypsin deficiency is a chronic, hereditary, usually
fatal, autosomal recessive disorder in which a low concentration of alpha1-PI
(alpha1-antitrypsin) is associated with slowly progressive, severe
panacinar emphysema that most often manifests itself in the third to fourth
decades of life.2-9 [Although the terms “alpha1-
Proteinase Inhibitor” and “alpha1-antitrypsin”
are used interchangeably in the scientific literature, the hereditary disorder
associated with a reduction in the serum level of alpha1-PI is conventionally
referred to as “alpha1- antitrypsin deficiency” while
the deficient protein is referred to as “Alpha1-Proteinase Inhibitor”10]. The emphysema is typically worse in the lower
lung zones.4,8,9 The pathogenesis of development of emphysema in
alpha1-antitrypsin deficiency is not well understood at this time.
It is believed, however, to be due to a chronic biochemical imbalance between
elastase (an enzyme capable of degrading elastin tissues, released by inflammatory
cells, primarily neutrophils, in the lower respiratory tract) and alpha1-PI
(the principal inhibitor of neutrophil elastase), which is deficient in alpha1-antitrypsin
disease.11-15 As a result, it is believed that alveolar structures
are unprotected from chronic exposure to elastase released from a chronic, low-level
burden of neutrophils in the lower respiratory tract, resulting in progressive
degradation of elastin tissues.11-15 The eventual outcome is the
development of emphysema. Neonatal hepatitis with cholestatic jaundice appears
in approximately 10% of newborns with alpha1-antitrypsin deficiency.15
In some adults, alpha1- antitrypsin deficiency is complicated by
cirrhosis.15
A large number of phenotypic variants of alpha1-antitrypsin deficiency
exists.15 The most severely affected individuals are those with the
PiZZ variant, typically characterized by alpha1-PI serum levels 35%normal.15
Epidemiologic studies of individuals with various phenotypes of alpha1-antitrypsin
deficiency have demonstrated that individuals with endogenous serum levels of
alpha1-PI 50 mg/dL (based on commercial standards) have a risk of
80% of developing emphysema over a lifetime.3-6,8,9,16 However, individuals
with endogenous alpha1-PI levels 80 mg/dL, in general,
do not manifest an increased risk for development of emphysema above the general
population background risk.5,15 From these observations, it is believed
that the “threshold” level of alpha1-PI in the serum
required to provide adequate anti-elastase activity in the lung of individuals
with alpha1-antitrypsin deficiency is about 80 mg/dL (based on commercial
standards for immunologic assay of alpha1-PI).12,15,17
In clinical studies of Alpha1-Proteinase Inhibitor (Human), Prolastin (alpha) ,
23 subjects with the PiZZ variant of congenital deficiency of alpha1-antitrypsin
deficiency and documented destructive lung disease participated in a study of
acute and/or chronic replacement therapy with Prolastin.18 The mean
in vivo recovery of alpha1-PI was 4.2 mg (immunologic)/dL per mg
(functional)/kg body weight administered.18,19 The half-life of alpha1-PI
in vivo was approximately 4.5 days.18,19 Based on these observations,
a program of chronic replacement therapy was developed. Nineteen of the subjects
in these studies received Prolastin (alpha) replacement therapy, 60 mg/kg body weight,
once weekly for up to 26 weeks (average 24 weeks of therapy). With this schedule
of replacement therapy, blood levels of alpha1-PI were maintained
above 80 mg/dL (based on the commercial standards for alpha1-PI immunologic
assay).18-20 Within a few weeks of commencing this program, bronchoalveolar
lavage studies demonstrated significantly increased levels of alpha1-PI
and functional antineutrophil elastase capacity in the epithelial lining fluid
of the lower respiratory tract of the lung, as compared to levels prior to commencing
the program of chronic replacement therapy with Alpha1-Proteinase
Inhibitor (Human), Prolastin (alpha) .18-20
All 23 individuals who participated in the investigations were immunized with
Hepatitis B Vaccine and received a single dose of Hepatitis B Immune Globulin
(Human) on entry into the investigation. Although no other steps were taken
to prevent hepatitis, neither hepatitis B nor non-A, non-B hepatitis occurred
in any of the subjects.18,19 All subjects remained seronegative for
HIV antibody. None of the subjects developed any detectable antibody to alpha1-PI
or other serum protein.
Long-term controlled clinical trials to evaluate the effect of chronic replacement
therapy with Prolastin (alpha) on the development of or progression of emphysema in
patients with congenital alpha1-antitrypsin deficiency have not been
performed. Estimates of the sample size required of this rare disorder and the
slow, progressive nature of the clinical course have been considered impediments
in the ability to conduct such a trial.21 Studies to monitor the
long-term effects will continue as part of the postapproval process.
REFERENCES
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18. Data on file.
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