Biomedical Engineering Reference
In-Depth Information
prodrug and drug pharmacokinetics to be carried out. Again,
a small proportion of the conjugate was radiolabeled to
allow
g
-camera images as well as to estimate enzyme
concentrations in the tumor over time. Quantitative SPECT
g
-camera images confirmed localization of the conjugate in
tumors. SB43gal effectively removed enzyme from blood. In
addition, tumor biopsies obtained after completion of
SB43gal and at the time of prodrug administration showed
median enzyme activity of 0.47 units per gram tumor (range
0.32-0.62), whereas normal liver obtained in biopsies from
two patients showed no detectable enzyme activity. These
findings confirmed the selectivity of the three-phase system
in that SB43gal eliminated enzyme from blood without
affecting tumor enzyme such that tumor to plasma enzyme
ratio of
the conjugate demonstrated efficacy in a two-component
system [39]. Based on these studies, a clinical trial of two-
phase system where the enzyme-inactivating antibody
SB43gal was omitted was initiated.
23.3.3 The Third Clinical Study of ADEPT
The trial was conducted to study the localization
and clearance of the monoclonal anti-CEA antibody
A5B7-F(ab
0
)
2
fragment conjugated to CPG2 without the
use of the clearing antibody, SB43gal. The study also
investigated safety and tolerability of the new BIP prodrug
in a two-phase system [40].
As no clearing agent was applied, it was decided that the
prodrug would be given when the enzyme activity in serum
was below a predefined value. Twenty-seven patients were
entered in this study. They were given a fixed dose of A5B7-
F(ab
0
)
2
-CPG2 conjugate, similar to the one used in the
previous clinical trials, followed by escalating dose of the
BIP prodrug when serum enzyme levels were below
0.05 units/mL. The time at which prodrug was administered
to patients ranged between 2 and 9 days. The maximum
tolerated dose of prodrug was reached at 15.5 mg/m
2
when
serum enzyme level was
<
0.05 units/mL. The BIP prodrug
at the MTD level showed elimination half-life of 14.5min.
The half-life of the drug was too short to be measured.
10,000:1 was observed at the time of prodrug
administration. Efficacy was demonstrated in some patients.
One patient with drug-resistant tumor growing through the
abdominal wall at a colostomy site had complete resolution
of that lesion and a partial response lasing 4 months. Six
patients had stable disease.
Pharmacokinetic studies demonstrated that the generated
drug had a median half-life of 46min (range 5-27min) [37],
which was sufficient to allow leak back from tumors causing
myelosuppression.
>
23.3.2.1 Conclusions from the Second Clinical Study of
ADEPT Conclusions from the second clinical study of
ADEPT are as follows:
23.3.3.1 Conclusions from the Third Clinical Study of
ADEPT Conclusions from the third clinical study of
ADEPT are as follows:
SB43gal eliminated enzyme from blood without affect-
ing concentration of tumor-located enzyme.
Tumor to plasma ratios of enzyme activity were
>
The tumor localization of the conjugate was poor.
The tumor to serum enzyme ratios were
1.
<
10,000:1 [36].
Tumor to liver ratios in two patients biopsied had ratio
>
There were no objective responses.
However, the study demonstrated that the new BIP
prodrug was safe in patients and provided valuable
information for future use.
10,000:1.
Enzyme measurements in tumor biopsies correlated
well with the
g
-camera estimates from radioactivity.
Drug half-life in plasma was longer than the prodrug
half-life [37] resulting in diffusion of drug into the
circulation.
There was evidence of antitumor response in patients
after only one ADEPT cycle.
23.4 FUSION PROTEINS
Our ADEPT studies indicated that it is desirable to use a
clearance system with the antibody-enzyme conjugates or
that a better targeting molecule needs to be developed.
Advances in the recombinant technology made it possible
to construct antibody-enzyme fusion proteins [41]. One of
the earliest recombinant fusion protein used in ADEPT was
an anti-CEA Fab fused to human
b
-glucuronidase [42].
Recombinant fusion proteins have advantages in terms of
reproducibility of the product as well as stability. We have
utilized a recombinant fusion protein comprising an anti-
CEA scFv antibody derived from an immunized mouse
phage library fused to CPG2 [43]. The construct was first
These studies confirmed the need for a drug with a short
half-life and a program of prodrug development was initi-
ated. Several new prodrugs were synthesized and a new
prodrug, 4-[N,N-bis(2-iodoethyl)amino]phenoxycarbonyl
L
-glutamic acid (short name BIP prodrug) was identified
for potential clinical studies [38]. BIP prodrug generated a
drug that was highly potent and with a shorter chemical and
biological half-life than the previous benzoic acid mustard
drug. Preclinical studies in the xenograft systems with the
conjugate followed by and the new BIP prodrug at 72 h after
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