Mechanism Of Action
CROFAB is a venom-specific Fab fragment of immunoglobulin G (IgG) that works by binding and
neutralizing venom toxins, facilitating their redistribution away from target tissues and their elimination
from the body.
The pharmacokinetic study of CROFAB was not adequately performed. A limited number of samples
were collected from three patients. Based on these data, estimates of elimination half-life were made.
The elimination half life for total Fab ranged from approximately 12 to 23 hours. These limited
pharmacokinetic estimates of half-life are augmented by data obtained with an analogous ovine Fab
product produced by Protherics Inc. using a similar production process. In that study, 8 healthy subjects
were given 1 mg of intravenous digoxin followed by an approximately equimolar neutralizing dose of
76 mg of digoxin immune Fab (ovine). Total Fab was shown to have a volume of distribution of 0.3
L/kg, a systemic clearance of 32 mL/min (approximately 0.4 mL/min/kg) and an elimination half-life of
approximately 15 hours.
Animal Toxicology And/Or Pharmacology
CROFAB was effective in neutralizing the venoms of 10 clinically important North American crotalid
snakes in a murine lethality model (see Table 4) . In addition, preliminary data from experiments in
mice using whole IgG from the sheep immunized for CROFAB production suggest that CROFAB might
possess antigenic cross-reactivity against the venoms of some Middle Eastern and North African
snakes, however, there are no clinical data available to confirm these findings.
Table 4: Average ED50 Values for CROFAB in Mice
|Study Objective & Design
||Major Findings and Conclus ions
|To determine the crossneutralizing
ability of CROFAB
to protect mice from the lethal
effects of venom from clinically
||ED50 for each venom
||(Note: Lower numbers represent increased potency against venoms listed)
||ED50 (mg antivenin/mg venom)
|C. h. atricaudatus
|C. v. helleri
|C. m. molossus
|A. c. contortrix
|S. m. barbouri
|C. h. horridus
|Separate groups of mice were injected with increasing doses of CROFAB pre-mixed with two LD of each venom tested.
||Based on data from studies in mice, CROFAB has relatively good cross-protection against venoms not used in the immunization of flocks used to
produce it. For C. v. helleri and C. m. molossus, higher doses may be required based on historical data.
No clinical studies have been conducted comparing CROFAB with other antivenins, therefore, no
comparisons can be made between CROFAB and other antivenins.
Two prospective clinical trials using CROFAB have been conducted. They were prospectively
defined, open label, multi-center trials conducted in otherwise healthy patients 11 years of age or older
who had suffered from minimal or moderate (as defined in Table 5) North American crotalid
envenomation that showed evidence of progression. Progression was defined as the worsening of any
evaluation parameter used in the grading of an envenomation: local injury, laboratory abnormality or
symptoms and signs attributable to crotalid snake venom poisoning. Both clinical trials excluded patients
with Copperhead envenomation.
Table 5: Definition of Minimal, Moderate, and Severe Envenomation in Clinical
Studies of CROFAB
||Swelling, pain, and ecchymosis limited to the immediate bite site;
Systemic signs and symptoms absent;
Coagulation parameters normal with no clinical evidence of bleeding.
||Swelling, pain, and ecchymosis involving less than a full extremity or,
if bite was sustained on the trunk, head or neck, extending less than 50
Systemic signs and symptoms may be present but not life threatening,
including but not limited to nausea, vomiting, oral paresthesia or unusual
tastes, mild hypotension (systolic blood pressure >90 mmHg), mild
tachycardia (heart rate <150), and tachypnea;
Coagulation parameters may be abnormal, but no clinical evidence of
bleeding present. Minor hematuria, gum bleeding and nosebleeds are
allowed if they are not considered severe in the investigator’s
||Swelling, pain, and ecchymosis involving more than an entire extremity
or threatening the airway;
Systemic signs and symptoms are markedly abnormal, including severe
alteration of mental status, severe hypotension, severe tachycardia,
tachypnea, or respiratory insufficiency;
Coagulation parameters are abnormal, with serious bleeding or severe
threat of bleeding.
In both clinical studies, efficacy was determined using a Snakebite Severity Score (SSS)  (referred to
as the efficacy score or ES in these clinical studies) and an investigator’s clinical assessment (ICA) of
efficacy. The SSS (referred to as the ES) is a tool used to measure the severity of envenomation based
on six body categories: local wound (e.g., pain, swelling and ecchymosis), pulmonary, cardiovascular,
gastrointestinal, hematological, and nervous system effects. A higher score indicates worse symptoms.
In a retrospective study using medical records of 108 snakebite victims , the SSS has been shown to
correlate well with physicians’ assessment of the patient's condition at presentation (Pearson correlation
coefficient: r=0.63, p<0.0001) and when the patient's condition was at its worst (r=0.70, p<0.0001). In
this study, the condition of 87/108 patients worsened during hospitalization. Changes in the physicians'
assessment of condition correlated well with changes in SSS. CROFAB was required to prevent an
increase in the ES in order to demonstrate efficacy.
The ICA was based on the investigator’s clinical judgment as to whether the patient had a:
- Clinical response (pre-treatment signs and symptoms of envenomation were arrested or improved
- Partial response (signs and symptoms of envenomation worsened, but at a slower rate than expected
- Non-response (the patient’s condition was not favorably affected by the treatment).
Safety was assessed by monitoring for early allergic events, such as anaphylaxis and early serum
reactions during CROFAB infusion, and late events, such as late serum reactions.
In the first clinical study of CROFAB, 11 patients received an intravenous dose of 4 vials of CROFAB
over 60 minutes. An additional 4-vial dose of CROFAB was administered after completion of the first
CROFAB infusion, if deemed necessary by the investigator. At the 1-hour assessment, 10 out of 11
patients had no change or a decrease in their ES. Ten of 11 patients were also judged to have a clinical
response by the ICA. Several patients, after initial clinical response, subsequently required additional
vials of CROFAB to stem progressive or recurrent symptoms and signs. No patient in this first study
experienced an anaphylactic or anaphylactoid response or evidence of an early or late serum reaction as
a result of administration of CROFAB.
Based on observations from the first study, the second clinical study of CROFAB compared two
different dosage schedules. Patients were given an initial intravenous dose of 6 vials of CROFAB with
an option to retreat with an additional 6 vials, if needed, to achieve initial control of the envenomation
syndrome. Initial control was defined as complete arrest of local manifestations, and return of
coagulation tests and systemic signs to normal. Once initial control was achieved, patients were
randomized to receive additional CROFAB either every 6 hours for 18 hours (Scheduled Group) or as
needed (PRN Group).
In this trial, CROFAB was administered safely to 31 patients with minimal or moderate crotalid
envenomation. All 31 patients enrolled in the study achieved initial control of their envenomation with
CROFAB, and 30, 25 and 26 of the 31 patients achieved a clinical response based on the ICA at 1, 6 and
12 hours respectively following initial control. Additionally, the mean ES was significantly decreased
across the patient groups by the 12-hour evaluation time point (p=0.05 for the Scheduled Group; p=0.05
for the PRN Group) (see Table 6). There was no statistically significant difference between the
Scheduled Group and the PRN Group with regard to the decrease in ES.
Table 6: Summary of Patient Efficacy Scores for Scheduled and PRN Groups
Mean ± SD
Mean ± SD
||4.0 ± 1.3
||4.7 ± 2.5
|End of Initial Control
|3.2 ± 1.4
||3.3 ± 1.3
|1 hour after Initial
|3.1 ± 1.3
||3.2 ± 0.9
|6 hours after Initial
|2.6 ± 1.5
||2.6 ± 1.3
|12 hours after Initial
|2.4 ± 1.1**
||2.4 ± 1.2**
|*No change or a decline in the Efficacy Score was considered an indication of clinical
response and a sign of efficacy.
**For both the Scheduled and the PRN Groups, differences in the Efficacy Score at the
four post-baseline assessment times were statistically decreased from baseline by Friedman’s test (p < 0.001).
In published literature accounts of rattlesnake bites, it has been noted that a decrease in platelets can
accompany moderately severe envenomation, which whole blood transfusions could not correct .
These platelet count decreases have been observed to last for many hours and often several days
following the venomous bite [3, 4, 5]. In this clinical study, 6 patients had pre-dosing platelet counts
below 100,000/mm3 (baseline average of 44,000/mm3). Of note, the platelet counts for all 6 patients
increased to normal levels (average 209,000/ mm3) at 1 hour following initial control dosing with
CROFAB (see Figure 1).
Figure 1 - Graph of Platelet Counts from Baseline to 36 Hours for Patients with Counts
<100,000/mm3 at Baseline (Study TAb002)
Although there was no significant difference in the decrease in ES between the two treatment groups,
the data suggest that Scheduled dosing may provide better control of envenomation symptoms caused by
the continued leaking of venom from depot sites. Scheduled patients experienced a lower incidence of
coagulation abnormalities at follow up compared with PRN patients (see Table 7 and Figure 2). In
addition, the need to administer additional CROFAB to patients in the PRN Group after initial control
suggests that there is a continued need for antivenin for adequate treatment.
Table 7 Lower Incidence of Recurrence of Coagulopathies at Follow-Up in
Scheduled and PRN Dos ing Groups
||Scheduled Group (n=14)*
(percent of patients with
|PRN Group (n=16)
(percent of patients with
|∧Numbers are expressed as percent of patients that had a follow-up platelet count that was
less than the count at hospital discharge, or a fibrinogen level less than 50% of the level
at hospital discharge.
*Follow-up data not available for one patient.
**Statistically significant difference, p=0.04 by Fisher’s Exact test.
Figure 2 - Change in Platelet Counts in Individual Patients between Follow-Up Visits and
Patients in the Scheduled and PRN Groups are plotted separately. More patients in the PRN Group
showed a reduction in platelet count after discharge than in the Scheduled Group. Only patients showing
a reduced platelet count after discharge are shown.
Following marketing approval of CROFAB a retrospective study was conducted to assess the efficacy
of CROFAB in severe envenomation. This study was a multi-center retrospective chart review of
medical records of snakebite patients treated with CROFAB and compared treatment and outcomes of
severe envenomations to those of mild and moderate envenomations. The primary efficacy variable was
severity of envenomation as determined by a 7-point severity score. Patients were classified as having
mild, moderate, or severe envenomation based on their scores just prior to receiving antivenom. Those
subjects with a severity score of 5 or 6 at the start of antivenom therapy were a priori defined as severe
envenomations; those with a score of 3 or 4 were defined as moderate envenomations, and those with a
score of 1 or 2 were defined as mild envenomations (see Table 5). A total of 247 patients of all
severities were included in the study. Patients with enough data to determine baseline severity were
included in the efficacy evaluation; this comprised a cohort of 209 patients, of which 28 were
classified as severe.
Improvement in the severity score was observed in all 28 severely envenomated patients. Improvement
was noted in every one of the severe venom effects studied, including limb pain and swelling,
cardiovascular, respiratory, gastrointestinal and neurologic effects, as well as
coagulopathy/defibrination syndrome, thrombocytopenia, and significant/spontaneous bleeding. The
median dose of CROFAB administered to control these severe venom effects was 9.0 vials (median of
2.0 doses). Initial control of envenomation was achieved in 57% (16/28) of severely envenomated
patients and 87% (158/181) of mild/moderate envenomated patients. In both groups failure to achieve
initial control was most commonly attributable to persistent coagulopathy and/or thrombocytopenia,
although medically significant bleeding has been reported (occurring in only 1 severe patient that did
not reach initial control). All 12 severe patients who did not reach initial control received only one
bolus dose of 4 to 6 vials to try to achieve initial control of envenomation. Of the 23 mild/moderate
cases who did not reach initial control, 19 did not follow recommended dosing for number of doses and
vials. Whether initial control could have been achieved with larger initial doses of antivenom cannot be
determined from this retrospective study. All patients, whether they achieved initial control or not,
experienced significant improvement of venom effects and decreased severity scores after receiving
CROFAB. Among the patients with severe envenomation who did not achieve initial control, the median
severity score improved from 5.0 (range: 5.0 – 6.0) before CROFAB administration to 2.0 (range: 1.0 –
4.0) at the last loading dose. No patient in this analysis had a severity score greater than 3.0 at the time
of final clinical assessment.
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