CLINICAL PHARMACOLOGY
Mechanism Of Action
The mechanism of action of defibrotide sodium has not
been fully elucidated. In vitro, defibrotide sodium enhances the enzymatic activity
of plasmin to hydrolyze fibrin clots. Studies evaluating the pharmacological
effects of defibrotide sodium on endothelial cells (ECs) were conducted
primarily in the human microvascular endothelial cell line. In vitro,
defibrotide sodium increased tissue plasminogen activator (t-PA) and
thrombomodulin expression, and decreased von Willebrand factor (vWF) and
plasminogen activator inhibitor-1 (PAI-1) expression, thereby reducing EC
activation and increasing EC-mediated fibrinolysis. Defibrotide sodium
protected ECs from damage caused by chemotherapy, tumor necrosis factor-α
(TNF-α), serum starvation, and perfusion.
Pharmacodynamics
Cardiac Electrophysiology
At a dose 2.4 times the maximum recommended dose,
DEFITELIO does not prolong the QTc interval to any clinically relevant extent.
PAI-1 Inhibition
Plasma concentrations of PAI-1 were assessed on an
exploratory basis as a potential pharmacodynamic marker for efficacy in Study
2. PAI-1 is an inhibitor of t-PA and therefore of fibrinolysis. Mean PAI-1
levels on Days 7 and 14 were lower than those at baseline in patients with
complete response (CR) and in those who were alive at Day+100, but this trend
did not reach statistical significance. There were no statistically significant
differences in mean PAI-1 levels by treatment or outcome.
Pharmacokinetics
Absorption
After intravenous administration, peak plasma
concentrations of defibrotide sodium occur approximately at the end of each
infusion.
Distribution
Defibrotide sodium is highly bound to human plasma
proteins (average 93%) and has a volume of distribution of 8.1 to 9.1 L.
Elimination
Metabolism followed by urinary excretion is likely the
main route of elimination. The estimated total clearance was 3.4 to 6.1 L/h.
The elimination half-life of defibrotide sodium is less than 2 hours. Similar
plasma concentration profiles were observed in VOD patients after initial and
multiple-dose administration of 6.25 mg/kg every 6 hours for 5 days. Therefore,
no accumulation is expected following multiple-dose administration.
Metabolism
Though the precise pathway of defibrotide sodium
degradation in plasma in vivo is largely unknown, it has been suggested that
nucleases, nucleotidases, nucleosidases, deaminases, and phosphorylases
metabolize polynucleotides progressively to oligonucleotides, nucleotides,
nucleosides, and then to the free 2'-deoxyribose sugar, purine and pyrimidine
bases.
The biotransformation of defibrotide sodium was
investigated in vitro by incubation with human hepatocytes from donors of
different ages and showed that defibrotide sodium does not undergo appreciable
metabolism by human hepatocyte cells.
Excretion
After administration of 6.25 mg/kg to 15 mg/kg doses of
DEFITELIO as 2-hour infusions, approximately 5-15% of the total dose was
excreted in urine as defibrotide sodium, with the majority excreted during the
first 4 hours.
Specific Populations
Age: Pediatric Population
Insufficient PK data were collected in pediatric patients
to draw conclusions.
Renal Impairment
The safety, tolerability, and pharmacokinetics of 6.25
mg/kg as 2-hour intravenous infusions of DEFITELIO were evaluated in patients
with Hemodialysis-dependent End Stage Renal Disease (ESRD) during hemodialysis
and on days off dialysis, and in patients with severe renal disease or ESRD not
requiring dialysis. Defibrotide sodium was not removed by hemodialysis, which
had no notable effect on plasma clearance of defibrotide sodium. Terminal half-lives
were consistently less than 2 hours, and there was no accumulation of
defibrotide sodium following repeated dosing. Defibrotide sodium exposure (AUC)
in patients with severe renal impairment or ESRD was 50% to 60% higher than
that observed in matched healthy subjects. Peak concentration (Cmax) was 35% to
37% higher following single- and multiple-dose administration of defibrotide
sodium.
Drug Interactions
Pharmacokinetic drug-drug interactions are unlikely at
therapeutic dose. Data from in vitro studies using human biomaterial demonstrate
that defibrotide sodium does not induce (CYP1A2, CYP2B6, CYP3A4, UGT1A1) or inhibit
(CYP1A2, CYP2B6, CYP3A4, CYP2C8, CYP2C9, CYP2C19, CYP2D6, UGT1A1, UGT2B7) the
major drug metabolizing enzymes and is not a substrate or inhibitor of the
major drug uptake transporters (OAT1, OAT3, OCT1, OCT2, OATP1B1, OATP1B3) or
efflux transporters (P-gp and BCRP).
There is some evidence (animal studies, ex vivo human
plasma, and healthy volunteers) that defibrotide sodium may enhance the
pharmacodynamic activity of heparin and alteplase [see DRUG INTERACTIONS].
Clinical Studies
The efficacy of DEFITELIO was investigated in three
studies: two prospective clinical trials (Study 1 and Study 2), and an expanded
access study (Study 3).
Study 1 enrolled 102 adult and pediatric patients in the
DEFITELIO treatment group with a diagnosis of VOD according to the following
criteria (bilirubin of at least 2 mg/dL and at least two of the following findings:
hepatomegaly, ascites, and weight gain greater than 5% by Day+21 post-HSCT)
with an associated diagnosis of multi-organ dysfunction (pulmonary, renal, or
both) by Day+28 post-HSCT. DEFITELIO was administered to the treatment group at
a dose of 6.25 mg/kg infused every 6 hours for a minimum of 21 days and
continued until patient was discharged from the hospital. Patients enrolled in
the DEFITELIO treatment group were not permitted to receive concomitant
medications such as heparin, warfarin, or alteplase because of an increased
risk of bleeding.
Study 2 included adult and pediatric patients with a
diagnosis of hepatic VOD and multi-organ dysfunction following HSCT, with 75
patients treated with DEFITELIO at a dose of 6.25 mg/kg infused every 6 hours.
The planned minimum duration of treatment was 14 days. The treatment could be
continued until signs of hepatic VOD resolved.
Study 3 is an expanded access program for DEFITELIO for
the treatment of adult and pediatric patients with hepatic VOD. The efficacy of
defibrotide was evaluated in 351 patients who had received a HSCT and developed
hepatic VOD with renal or pulmonary dysfunction. All patients received
DEFITELIO at a dose of 6.25 mg/kg infused every 6 hours.
Baseline demographic information and details for patients
treated in these studies are provided below in Table 3.
Table 3: Baseline Demographics of Patients Treated
with DEFITELIO at 6.25 mg/kg Every 6 Hours
Data Source |
Study 1 |
Study 2 |
Study 3 |
Design |
Prospective |
Prospective |
Expanded Access Study |
Number of patients |
102 |
75 |
351 |
Median age (years) (range) |
21 years ( < 1,72) |
32 years ( < 1, 61) |
15 years ( < 1, 69) |
Age, n (%) |
< 17 years |
44 (43%) |
22 (29%) |
189 (54%) |
≥ 17 years |
58 (57%) |
53 (71%) |
162 (46%) |
Race, n (%) |
White |
77 (75%) |
61 (81%) |
237 (68%) |
Black/African |
6 (6%) |
6 (8%) |
21 (6%) |
American Asian |
4 (4%) |
2 (3%) |
15 (4%) |
Other |
15 (15%) |
6 (8%) |
78 (22%) |
Gender, n (%) |
Male |
64 (63%) |
41 (55%) |
184 (52%) |
Female |
38 (37%) |
34 (45%) |
167 (48% ) |
Median number of days on treatment (days) (range) |
21.5 days (1,58) |
19.5 days (3,83) |
21.0 daysa (1,93) |
Type of graft, n (%) |
Allograft |
90 (88%) |
67 (89%) |
317 (90%) |
Autograft |
12 (12%) |
8 (11%) |
34 (10%) |
Ventilator or Dialysis Dependent at Study Entry, n (%) |
34 (33%) |
8 (11%) |
149 (42%) |
a Duration of treatment from first dose to
last dose is presented because days without treatment were not captured for the
expanded access study. |
The efficacy of DEFITELIO was based on survival at Day +
100 after HSCT. In Study 1, the survival rate was 38% (95% CI: 29%, 48%) at 100
days after transplantation. In Study 2 the survival rate was 44% (95% CI: 33%,
55%) at 100 days after transplantation. In Study 3, the Day + 100 survival was
45% (95% CI: 40%, 51%).
Based on published reports and analyses of patient level
data for individuals with hepatic VOD with renal or pulmonary dysfunction who
received supportive care or interventions other than DEFITELIO, the expected Day
+100 survival rates are 21% to 31%.