Pharmacological properties : תכונות פרמקולוגיות

Pharmacodynamic Properties

5.1     Pharmacodynamic properties

Pharmacotherapeutic group: Antineoplastic agents, antimetabolites, ATC code:L01BB06 
Mechanism of action-

Clofarabine is a purine nucleoside anti-metabolite. Its antitumour activity is believed to be due to 3 mechanisms:

•     DNA polymerase  inhibition resulting in termination of DNA chain elongation and/or DNA synthesis / repair.
•     Ribonucleotide reductase inhibition with reduction of cellular deoxynucleotide triphosphate (dNTP) pools.
•     Disruption of mitochondrial membrane integrity with the release of cytochrome C and other proapoptotic factors leading to programmed cell death even in non-dividing lymphocytes.

Clofarabine must first diffuse or be transported into target cells where it is sequentially phosphorylated to the mono- and bi-phosphate by intracellular kinases, and then finally to the active conjugate, clofarabine 5’-triphosphate. Clofarabine has high affinity for one of the activating phosphorylating enzymes, deoxycytidine kinase, which exceeds that of the natural substrate, deoxycytidine.

In addition, clofarabine possesses greater resistance to cellular degradation by adenosine deaminase and decreased susceptibility to phosphorolytic cleavage than other active substances in its class whilst the affinity of clofarabine triphosphate for DNA polymerase  and ribonucleotide reductase is similar to or greater than that of deoxyadenosine triphosphate.

Pharmacodynamic effects

In vitro studies have demonstrated that clofarabine inhibits cell growth in and is cytotoxic to a variety of rapidly proliferating haematological and solid tumour cell lines. It was also active against quiescent lymphocytes and macrophages. In addition, clofarabine delayed tumour growth and, in some cases, caused tumour regression in an assortment of human and murine tumour xenografts implanted in mice.

Clinical efficacy and safety

Clinical efficacy: To enable systematic evaluation of the responses seen in patients, an unblinded Independent Response Review Panel (IRRP) determined the following response rates based on definitions produced by the Children’s Oncology Group:

CR = Complete Remission                     Patients who met each of the following criteria: • No evidence of circulating blasts or extramedullary disease
• An M1 bone marrow (≤ 5% blasts)
• Recovery of peripheral counts (platelets  100 x 109/l and ANC  1.0 x 109/l)
CRp = Complete Remission in the             • Patients who met all of the criteria for a CR except Absence of Total Platelet Recovery            for recovery of platelet counts to > 100 x 109/l PR = Partial Remission                      Patients who met each of the following criteria: • Complete disappearance of circulating blasts
• An M2 bone marrow ( 5% and ≤ 25% blasts) and appearance of normal progenitor cells
• An M1 marrow that did not qualify for CR or CRp
Overall Remission (OR) Rate                 - (Number of patients with a CR + Number of patients with a CRp) ÷ Number of eligible patients who received clofarabine

The safety and efficacy of clofarabine were evaluated in a phase I, open-label, non-comparative, dose-escalation study in 25 paediatric patients with relapsed or refractory leukaemia (17 ALL; 8 AML) who had failed standard therapy or for whom no other therapy existed. Dosing commenced at 11.25 with escalation to 15, 30, 40, 52 and 70 mg/m2/day by intravenous infusion for 5 days every 2 to 6 weeks depending on toxicity and response. Nine of 17 ALL patients were treated with clofarabine 52 mg/m2/day. Of the 17 ALL patients, 2 achieved a complete remission (12%; CR) and 2 a partial remission (12%; PR) at varying doses. Dose-limiting toxicities in this study were hyperbilirubinaemia, elevated transaminase levels and maculo-papular rash experienced at 70 mg/m2/day (2 ALL patients; see section 4.9).

A multi-centre, phase II, open-label, non-comparative study of clofarabine was conducted to determine the overall remission (OR) rate in heavily pretreated patients (≤ 21 years old at initial diagnosis) with relapsed or refractory ALL defined using the French-American-British classification.
The maximum tolerated dose identified in the phase I study described above of 52 mg/m2/day clofarabine was administered by intravenous infusion for 5 consecutive days every 2 to 6 weeks. The table below summarises the key efficacy results for this study.

Patients with ALL must not have been eligible for therapy of higher curative potential and must have been in second or subsequent relapse and/or refractory i.e. failed to achieve remission after at least two prior regimens. Before enrolling in the trial, 58 of the 61 patients (95%) had received 2 to 4 different induction regimens and 18/61 (30%) of these patients had undergone at least 1 prior haematological stem cell transplant (HSCT). The median age of treated patients (37 males, 24 females) was 12 years old.

Administration of clofarabine resulted in a dramatic and rapid reduction in peripheral leukaemia cells in 31 of the 33 patients (94%) who had a measurable absolute blast count at baseline. The 12 patients who achieved an overall remission (CR + CRp) had a median survival time of 66.6 weeks as of the data collection cut-off date. Responses were seen in different immunophenotypes of ALL, including 
pre-B cell and T-cell. Although transplantation rate was not a study endpoint, 10/61 patients (16%) went on to receive a HSCT after treatment with clofarabine (3 after achieving a CR, 2 after a CRp, 3 after a PR, 1 patient that was considered a treatment failure by the IRRP and 1 that was considered not evaluable by the IRRP). Response durations are confounded in patients who received a HSCT.

Efficacy results from the pivotal study in patients (≤ 21 years old at initial diagnosis) with relapsed or refractory ALL after at least two prior regimens
Response              ITT*      Median duration            Median time to         Median overall category            patients      of remission               progression         survival (weeks) (n = 61)          (weeks)                 (weeks)**              (95% CI) (95% CI)                  (95% CI)
Overall                 12              32.0                     38.2                   69.5 remission             (20%)        (9.7 to 47.9)            (15.4 to 56.1)           (58.6 to -) (CR + CRp)
CR                       7              47.9                     56.1                   72.4 (12%)          (6.1 to -)               (13.7 to -)            (66.6 to -) CRp                      5              28.6                     37.0                   53.7 (8%)        (4.6 to 38.3)              (9.1 to 42)             (9.1 to -) PR                       6              11.0                     14.4                   33.0 (10%)          (5.0 to -)                (7.0 to -)            (18.1 to -) CR + CRp + PR           18              21.5                     28.7                   66.6 (30%)        (7.6 to 47.9)            (13.7 to 56.1)           (42.0 to -) Treatment               33              N/A failure               (54%)                                       4.0                    7.6 Not evaluable           10              N/A                  (3.4 to 5.1)           (6.7 to 12.6) (16%)
All patients            61              N/A                       5.4                   12.9 (100%)                                  (4.0 to 6.1)           (7.9 to 18.1) *ITT = intention to treat.
**Patients alive and in remission at the time of last follow up were censored at that time point for the analysis.



Individual duration remission and survival data for patients who achieved CR or CRp1 
Duration of          Overall
Time to OR Remission                   Survival
Best Response      (weeks)           (weeks)              (weeks)
Patients who did not undergo transplant
CR                 5.7               4.3                  66.6
CR                 14.3              6.1                  58.6
CR                 8.3               47.9                 66.6
CRp                4.6               4.6                  9.1
CR                 3.3               58.6                 72.4
CRp                3.7               11.7                 53.7
Patients who underwent transplant while in continued remission*
CRp                8.4              11.6+                 145.1+
CR                 4.1               9.0+                 111.9+
CRp                3.7               5.6+                 42.0
CR                 7.6               3.7+                 96.3+
Patients who underwent transplant after alternative therapy or relapse*
CRp                4.0              35.4                  113.3+**
CR                 4.0               9.7                  89.4***
* Duration of remission censored at the time of transplant
** Patient received a transplant following alternate therapy
*** Patient received a transplant following relapse

.

Pharmacokinetic Properties

5.2    Pharmacokinetic properties

Adsorption and distribution

Pharmacokinetics of clofarabine were studied in 40 patients aged between 2 to 19 years old with relapsed or refractory ALL or AML. Patients were enrolled into a single phase I (n = 12) or two phase II (n = 14 / n = 14) safety and efficacy studies, and received multiple doses of clofarabine by intravenous infusion (see section 5.1).



Pharmacokinetics in patients aged between 2 to 19 years old with relapsed or refractory ALL or AML following administration of multiple doses of clofarabine by intravenous infusion
Parameter                               Estimates based on        Estimates based on other non-compartmental                   analysis analysis
(n = 14 / n = 14)
Distribution:
Volume of distribution (steady                172 l/m2 state)
Plasma protein binding                                                      47.1% Serum albumin                                                               27.0% Elimination:
β half-life of clofarabine                   5.2 hours
Half-life of clofarabine triphosphate                                     > 24 hours Systemic clearance                          28.8 l/h/m2
Renal clearance                             10.8 l/h/m2
Dose excreted in urine                          57%

Multivariate analysis showed that the pharmacokinetics of clofarabine are weight dependent and although white blood cell (WBC) count was identified as having an impact on clofarabine pharmacokinetics, this did not appear sufficient to individualise a patient’s dosage regimen based on their WBC count. Intravenous infusion of 52 mg/m2 clofarabine produced equivalent exposure across a wide range of weights. However, Cmax is inversely proportional to patient weight and, therefore, small children may have a higher Cmax at the end of infusion than a typical 40 kg child given the same dose of clofarabine per m2. Accordingly, longer infusion times should be considered in children weighing  20 kg (see section 4.2).

Biotransformation and elimination

Clofarabine is eliminated by a combination of renal and non-renal excretion. After 24 hours, about 60% of the dose is excreted unchanged in the urine. Clofarabine clearance rates appear to be much higher than glomerular filtration rates suggesting filtration and tubular secretion as kidney elimination mechanisms. However, as clofarabine is not detectably metabolised by the cytochrome P450 (CYP) enzyme system, pathways of non-renal elimination currently remain unknown.

No apparent difference in pharmacokinetics was observed between patients with ALL or AML, or between males and females.

No relationship between clofarabine or clofarabine triphosphate exposure and either efficacy or toxicity has been established in this population.

Special populations

Adults (> 21 and < 65 years old)
There are currently insufficient data to establish the safety and efficacy of clofarabine in adult patients.
However, the pharmacokinetics of clofarabine in adults with relapsed or refractory AML following administration of a single dose of 40 mg/m2 clofarabine by intravenous infusion over 1 hour were comparable to those described above in patients aged between 2 to 19 years old with relapsed or refractory ALL or AML following administration of 52 mg/m2 clofarabine by intravenous infusion over 2 hours for 5 consecutive days.

Elderly patients (≥ 65 years old)
There are currently insufficient data to establish the safety and efficacy of clofarabine in patients 65 years of age or older.


Renal impairment
To date, there are limited data on the pharmacokinetics of clofarabine in paediatric patients with decreased creatinine clearance. However, these data indicate that clofarabine may accumulate in such patients (see figure below).

Population pharmacokinetic data from adult and paediatric patients suggest that patients with stable moderate renal impairment (creatinine clearance 30 – <60 ml/min) receiving a 50% dose reduction achieve similar clofarabine exposure to those with normal renal function receiving a standard dose.

Clofarabine AUC0-24 hours by baseline estimated creatinine clearance in patients aged between      2 to 19 years old with relapsed or refractory ALL or AML (n = 11 / n = 12) following administration of multiple doses of clofarabine by intravenous infusion (creatinine clearance estimated using Schwartz formula)
3000



Clofarabine     2500

AUC 0-24 hours
(ng*h/ml)      2000


1500


1000


500


0
0       50            100             150                 200   250

Estimated creatinine clearance (ml/min)



Hepatic impairment
There is no experience in patients with hepatic impairment (serum bilirubin > 1.5 x ULN plus AST and ALT > 5 x ULN) and the liver is a potential target organ for toxicity (see sections 4.3 and 4.4).