CLIN. CHEM. 38/10, 1930-1932
(1992)
Prostate-Specific Antigen Comes of Age in Diagnosis and Management of Prostate
Cancer
In the 6 years since the introduction
of the first
commercial immunoassays
for prostate-specific
antigen
(PSA), this unique tissue-specific
antigen has become
established
as the most useful serological
marker
for
monitoring
patients with prostate cancer. The antigen
was originally
isolated from semen by three independent laboratories
searching for semen-specific
antigens
(1-3). Later, at Roswell Park Institute, the antigen was
purified from normal prostate tissue, its natural source
(4). This research
group pioneered
the use of PSA as a
tumor marker to monitor patients with prostate cancer
(5).
Subsequent
experience
has shown that PSA is considerably more useful than prostatic acid phosphatase
to
monitor prostate
cancer (6, 7). Serum concentrations
of
PSA correlate strongly with the volume and clinical
stage of prostate cancer. After radical prostatectomy
for
localized cancer (stages A and B), serum PSA falls to
amounts
that are undetectable
by the current generation of immunoassays
for PSA (7). Serially
increasing
concentrations
of PSA above the limits of detection of
current tests appear to be the earliest sign of residual
cancer (7-11). Many physicians no longer use prostatic
acid phosphatase
to routinely monitor their cancer patients, although rare cases of patients with metastatic
prostate
cancer and normal PSA concentrations
have
been published (12).
More recently,
PSA has been recommended
as an
important
adjunct to a careful digital rectal examination to screen men over age 50 years for prostate cancer
(13-15). In these studies, approximately
one-third of the
men (range 30-35%) with increased PSA values (>4
g(L by Tandem-R PSA) had prostate cancer on rectal
biopsy. In contrast, in patients with abnormal prostates
by rectal examination
and positive biopsies for prostate
cancer, between
17% and 23% had PSA values in the
normal range (<4 /Lg/L) (13-15). Because of this overlap, PSA should never be used as the sole screening test
for cancer. Other common prostate conditions such as
benign prostatic hypertrophy
(BPH) and prostatitis can
produce increases in serum PSA similar to those seen in
cancer patients. In patients with increased PSA concentrations on screening,
only a careful digital rectal examination coupled with a rectal ultrasound evaluation,
with systematic
as well as directed biopsy of suspicious
hypoechoic areas, can distinguish BPH from cancer (16).
A recent retrospective
serological study showed that the
rate of increase
in PSA in serial determinations
was
useful for distinguishing
early prostate cancer from
BPH (17). Patients with prostate cancer showed a more
1930
CLINICAL CHEMISTRY, Vol. 38, No. 10, 1992
rapid increase in their PSA (>0.75 g/L per year) than
did patients with BPH, at a specificity of 90%. These
serological
changes occurred -5 years before clinical
diagnosis. That study greatly strengthens
the argument
for periodic screening
of men older than age 50 years
with a digital rectal examination
and a determination
of
serum PSA, to detect prostate cancer at a stage when it
is still potentially curable (18, 19). The use of PSA as a
screening
test to detect early prostate cancer will substantially increase the demand for this test.
In the current issue of Clinical Chemistry,
Vessella et
al. (20) and Dnistrian et al. (21) describe clinical testing
of a second-generation
test for PSA, the Abbott IMx PSA
test. This test offers the advantages
of an automated
microprocessor-controlled
analyzer, which also provides
various other clinically important immunoassays,
e.g.,
serum (3-fragment of human chorionic gonadotropin,
a-fetoprotein,
and carcinoembryonic
antigen. In addition, the IMx PSA offers the convenience
of a short assay
time, calibration
curve storage, and an enzyme label
with a fluorescent substrate, thereby avoiding radioactive reagents.
In their extensive
comparative
studies of samples
from normal patients and patients with prostate cancer
and benign prostatic hypertrophy,
Vessella et al. demonstrated that the IMx PSA yielded results essentially
identical to the Hybritech Tandem-R test, a widely used
immunoradiometric
test for PSA licensed
by the US
Food and Drug Administration.
In a smaller study of
serum samples from patients not selected for prostate
pathology, Dnistrian et al. (21) similarly found that the
IMx PSA values were essentially
identical
to Hybritech’s immunoenzymometric
assay for PSA, the Tandem-E PSA assay. This is not surprising,
because the
manufacturer
calibrated
the IMx assay against
the
Tandem-R PSA calibrators. From the practical
point of
view, laboratories
assaying
many PSA samples may
find the 23-tube limit per run (plus one calibrator) of the
IMx PSA inconvenient,
particularly
in view of the
projected large volume of PSA testing that will be
required if PSA is widely accepted as a cancer screening
test in the coming years. Nonetheless,
as an automated
PSA immunoassay
that yields PSA values highly comparable with those of the Tandem-Rand
Tandem-E PSA
tests, the IMx PSA is likely to find widespread
acceptance in clinical laboratories.
Vessella et al. (20) make an additional claim for the
JMx PSA that is less well documented, i.e., that it has a
significantly
lower biological detection limit than the
Tandem-R test and therefore can detect residual cancer
at an earlier stage after radical prostatectomy.
In contrast, the Dnistrian group found that the IMx PSA and
the Tandem-E
PSA tests had similar lower limits of
detection
(21). The concept of the biological
detection
limit introduced
by Vessella et al. (20) is an appealing
one, because this measure of assay sensitivity
takes into
account both the analytical
sensitivity
(lower limit of
detection) of an assay as well as the additional variability caused by the biological
matrix
(e.g., serum)
in
which the analyte (PSA) is found. Nevertheless,
the
evidence presented here is contradictory
as to whether
the Abbott IMx PSA offers any significant
advantage
in
sensitivity
over the Hybritech
Tandem-R
and Tandem-E tests.
The potential usefulness of an ultrasensitive
PSA test
to detect residual cancer at a very early stage after
radical prostatectomy
was recently
reported by our
Stanford
research
group (22). That study describes development
of a dedicated well-controlled
ultrasensitive
PSA radioimmunoassay
capable of detecting
residual
cancer on an average of --10 months earlier than the
current generation
of tests (22).’ Earlier detection of
residual cancer was possible because of the observation
that patients after prostatectomy
with no evidence of
cancer always had PSA concentrations
<0.1 gfL by
this ultrasensitive
test. Patients with residual cancer
showed progressive
increases in PSA >0.1 gfL (22).
Vessella et al. (20) confirmed these findings here. We
call the PSA value of 0.1 g/L
the residual
cancer
detection limit’, similar to the term clinical threshold
used by Vessella et al. (20).
However, the current lMx PSA test lacks the necessary calibrators
and serum PSA controls to ensure
reliable results in the ultrasensitive
range. Vessella et
al. suggest that the manufacturer
provide at least two
external PSA curve controls in the ultrasensitive
range
(i.e., PSA <0.5 gfL). In addition, the IMx PSA should
also include several PSA calibrators in the ultra-lowconcentration
range to qualifr as an ultrasensitive
test.
As a rule of thumb, assays should have at least one
calibrator per log,0 unit interval over the working assay
range. Currently, the IMx PSA has no PSA calibrators
between
0.03 (the lower limit of detection of the test)
and 2.0 tg/L, a concentration
difference of almost 70fold. The sole purpose of an ultrasensitive
PSA assay is
to diagnose residual cancer at the earliest possible stage
after prostatectomy
(22).’ Thus, extra quality-control
samples should be included in this assay.
There is currently no proven effective adjuvant therapy for treating prostate cancer at very early stages.
Care must be used, therefore, in making the diagnosis of
residual cancer based solely on PSA concentrations.
In
postprostatectomy
patients with PSA values increasing
to >0.1 giL
(the residual cancer detection limit or
clinical threshold),
serial PSA values should be moth-
‘Stamey TA, Graves HCB, Wehner N, Ferrari M, Freiha F.
Early detection of residual prostate cancer after radical prostatectomy by an ultraaensitive
assay for prostate-specific
antigen. J
Urol 1992 (submitted).
tored until a progressive linear increase in PSA concentrations above this value is clearly established
by an
ultrasensitive
PSA assay.’ A single PSA >0.1 p.g/L
indicates a high probability of residual cancer, but does
not prove that cancer is invariably
present. Once a
diagnosis of residual
cancer has been established,
the
availability
of well-controlled
ultrasensitive
PSA tests
in the near future will allow experimental
clinical trials
to proceed to test the effectiveness
of various adjuvant
regimens for treating
very early residual disease.1
Clinical chemists should look for the following critical
control features in future ultrasensitive
PSA assays: (a)
several PSA calibrators
in the ultrasensitive
range
(<0.5 pgfL), ideally with at least one calibrator near 0.1
zgfL; and (b) multiple PSA controls (ideally, human
serum samples) in the low-concentration
range to control for interassay
variation
(e.g., two samples
<0.3
p.g/L, with one sample close to the 0.1 g/L detection
limit for residual cancer (22). As an extra precaution,
samples with PSA values at or near 0.1 p.g/L should
probably be analyzed in duplicate by the ultrasensitive
tests. These assay features are necessary
to meet the
need for adequate
control at diagnostically
critical concentrations,
a principle accepted by clinical biochemists
and regulatory
agencies.
Because of these special requirements,
it may be best for manufacturers
to offer
two PSA assays, i.e., a standard
one for screening
purposes
and for monitoring
prostate cancer patients
with established
disease (working
range >0.3 pg/L),
and a separate
well-controlled
ultrasensitive
assay to
monitor patients after radical prostatectomy
for detecting early evidence of residual
disease (working range
>0.05 zg/L).
With the rapidly expanding
clinical
usefulness
of
PSA, laboratory clinicians can expect to see a variety of
new PSA tests in the coming years. However, issues of
major importance remain to be resolved in this rapidly
evolving field. First, there is a need for an international
antigen standard
(23). Several of the current generation
of immunoassays
of PSA differ considerably
in their
values when used to test the same patients’
samples
(23-25). Much of this difference
appears
to be due to
differences
in the assigned protein values of the PSA
used to calibrate
the tests (23, 25); adoption
of an
international
antigen standard
would help resolve this
issue. Second, PSA is a chymotrypsin-like
serine protease with sequence relatedness
to the kallikrein
family
(26). Most PSA in serum is bound to serum proteinase
inhibitors, especially a,-antichymotrypsin
and a2-macroglobulin (27,28). The binding of PSA to these macromolecules in serum may interfere with antibody binding,
especially
in
solid-phase
assays
utilizing
monoclonal
antibodies.
Use of a defined matrix diluent
containing
these natural
proteinase
inhibitors
may be
necessary
for standardizing
different
assays based on
different monoclonal and polyclonal antibodies. In view
of the evidence that monitoring serial PSA values over
time may have clinical usefulness for early detection of
prostate cancer (17), the need to calibrate the different
commercial
PSA tests to the same antigen standards
CLINICAL CHEMISTRY, Vol. 38, No. 10, 1992
1931
and diluent matrices
is critical. Values obtained by
different PSA assay methodologies
cannot be used interchangeably
at the current time.
Early resolution of these assay standardization
issues
coupled with the increased availability
of a new generation of rapid, automated, nonisotopic PSA assays such
as the IMx PSA described
in this issue of Clinical
Chemistry,
both in the standard
and the ultrasensitive
formats, will provide a powerful and convenient tool for
clinicians
in diagnosing
and managing
patients with
prostate cancer.
I thank T. A. Stamey
work.
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Howard
Department
of Urology,
S287
Stanford
University
Medical Center
300 Pasteur Drive
Stanford,CA 94305
C. B. Graves