Phase II Trial of Pentostatin and Targeted Busulfan as a Novel Reduced Intensity Regimen for Allogeneic Hematopoietic Stem Cell Transplantation Using Laboratory-Guided (CD4-guided) Immunosuppression.

Trial Information

Phase II Trial of Pentostatin and Targeted Busulfan as a Novel Reduced Intensity Regimen for Allogeneic Hematopoietic Stem Cell Transplantation Using Laboratory-Guided (CD4-guided) Immunosuppression.

Pentostatin (deoxycoformycin) is a purine analoge that is currently indicated for the
treatment of chemo-naïve or interferon-refractory hairy cell leukemia. Pentostatin
represents an ideal agent for conditioning recipients before allogeneic hematopoietic cell
transplantation (HCT) due to its significant effect on lymphocytes and relatively reduced
myelosuppression as compared to other purine analogues. It is a potent inhibitor of the
enzyme adenosine deaminase (ADA) whose activity is generally more pronounced in T
lymphocytes as compared to their B counterparts. Also, pentostatin has been shown, though
its immunomodulatory effect to be active in patients with steroid-refractory aGVHD.

Pentostatin-based regimens have been studied as a conditioning strategy for patients
undergoing allogeneic stem cell transplantation. Chan et al. evaluated 18 MDS patients
ineligible for standard allogeneic transplantation. In that series, patients received
conditioning with a preparative regimen of photopheresis day -7 and -6, pentostatin 4 mg/m2
by continuous infusion day -5 and -4, and total body irradiation 600 cGy in 3 fractions day
-3 and -2, followed by allogeneic stem cell infusion from 6/6 or 5/6 HLA-matched related
donors or 6/6 HLA-matched unrelated donors. Sixteen of 18 (89%) patients developed full
donor chimerism, with no day +100 transplant-related mortality (TRM). Grade 2 to 4 acute
GVHD and extensive chronic GVHD developed in 19% and 18% of patients, respectively. Disease
relapse occurred in 2 patients. At a median follow-up of 14 months (range, 1-35 months), the
1-year failure-free and overall survival were 64% and 65%, respectively. These findings
clearly suggest that Pentostatin-based regimens are capable of achieving a successful donor
engraftment and durable disease-remission with significantly lesser transplant toxicity and
grade 2 to 4 acute GVHD. Pavletic et al, demonstrated that Pentostatin induces significant
lympho depleting effects with a promising safety, supporting its use as a conditioning
regimen in non-myeloablative strategies with hematopoietic cell infusions as early as 7 days
after initiation of Pentostatin.

Busulfan was available primarily as an oral formulation until recently. The oral drug has
variable bioavailability that has been linked to erratic gastrointestinal absorption from
patient to patient. In a phase I trial, Andersson et al evaluated an IV Bu formulation using
dimethylacetamide and PEG400 as the solvent. This is the formulation currently marketed in
the USA (Busulfex ESP Pharma). Fifteen patients with advanced hematologic malignancies were
treated with BU every 6 hours for a total of 16 doses followed by Cy 60mg/kg for 2 doses and
hematopoietic cell transplant. The first BU dose given was by IV. The starting IV dose was
0.08mg/kg which was then escalated to 0.2, 0.4, and finally to 0.8mg/kg in cohorts of 3
patients each. Six hours after the start of the IV BU infusion, the patients began to
receive oral BU 1mg/kg every 6 hours for 15 doses. Pharmacokinetic profiles were obtained
to determine an equivalent exposure of IV BU to the oral BU dose of 1mg/kg. It was noted
that the toxicity profile and rates of engraftment were similar to that of the oral drug
(i.e., no additional toxicities were found attributable to the solvents). An IV dose of
0.8mg/kg gave an equivalent AUC (but with less variability) as the oral dose of 1mg/kg with
a mean AUC of 1189uM-min (range 964-1547uM-min; 24h DIE 4756uM-min [range 3856-6188uM-min]).
Thus, 0.8mg/kg was chosen by the investigators as the dose to proceed to phase II testing. A
subsequent phase II multicenter trial in 61 patients with a variety of hematologic
malignancies using 0.8mg/kg every 6 hours x 16 doses of IV BU and 120mg/kg CY prior to
hematopoietic stem cell transplant. Pharmacokinetic analyses were also done. Once again, it
was shown that the toxicity and outcome data was similar to that published for the oral BU.
There were no patients with seizures. Lung toxicities occurred in two patients, one with
diffuse alveolar hemorrhage (DAH) and one with interstitial pneumonitis (IP); the latter
patient had a history of lung irradiation. Grade 1-2 nausea and emesis occurred in 43% and
grade 3 in 7%. Mucositis was grade 2 in 44%, Grade 3 in 26% and lasted a median of 6 days.
There were five (8%) cases of VOD, which was fatal in two (3%). Treatment related mortality
at 100 days was 9.8%. The AUC was <1500 uM-min (24h DIE <6000uM-min) in 55%; 86% of patients
maintained an AUC between 800 and 1500uM-min (24h DIE 3200-6000uM-min). The AUC at dose 1
and dose 9 were similar indicating a consistent dose-to-dose behavior. The authors concluded
that IV BU in this setting was well tolerated and demonstrated excellent antitumor efficacy,
likely resulting from predictable pharmacokinetics. Based on these and other studies, IV
administration has become the preferred route for BU when given in the high dose transplant
setting. Fernandez, et al. evaluated the use of IV Bu (Busulfex® ESP Pharma) in either a
once daily or twice daily dosing schedule when given as a part of BUCY regimen (BU: 3.2
mg/kg and CY 120mg/kg) prior to transplant. In both dosing schedules, there was little
variability in the dose-to-dose levels and the pharmacokinetics of the first dose could
predict the data on subsequent doses. In the twice-daily group, AUC 3390uM-min (2400-
4678uM-min; 24h DIE 6780uM-min [4800-9356uM-min]) and the once daily group the AUC
5561uM-min (4412 - 7368uM-min). The pharmacokinetic profiles (AUC, CL, Cmax and t 1/2) were
similar from dose to dose. In the twelve patients studied, the following grade 3 toxicities
were reported: mucositis, anorexia, infection, epistaxis and hyperglycemia. Two patients
died: one from sepsis from presumed fungal infection and one sudden cardiac death. One
patient developed mild, reversible VOD. Daily dosing of IV Bu results in similar outcomes
and toxicity profile compared to oral dosing and ease in delivering a targeted exposure of
BU. The importance of being able to target BU exposure has been illustrated in the above
discussion of reports correlating BU pharmacokinetic parameters with its toxicity and
efficacy. Thus, by utilizing a once daily IV dose, targeted to prespecified levels, BU
therapy can be optimized to limit toxicity and maximize efficacy.

To see if the AUC of IV Bu was correlated to adverse effects, Andersson and colleagues
treated 36 CML patients with either fixed or adjusted dose BuCy. The first 25 patients were
given a fixed dose of Bu 0.8mg/kg every 6 hours. In the subsequent 12 patients, doses were
adjusted to achieve an AUC of 1250uM-min (24h DIE 5000uM-min). At steady state, the
population median AUC was 1265uM-min (range, 816-1905uM-min [24h DIE 5056uM-min;
3264-7620uM-min]). They found that the probabilities of developing gastrointestinal
toxicity, hepatotoxicity, mucositis, and acute GVHD all increased with increasing AUC. The
26 patients within AUC of 950 - 1520uM-min (24h DIE 3800-6080uM-min) had a decreased
incidence of death compared with the 10 patients outside this range. There were no cases of
hepatic veno-occlusive disease or seizures. These were small numbers but did indicate a
trend towards achieving an optimal systemic exposure by delivering individualized doses and
a suggestion of a "therapeutic window" for Bu in this setting. The importance of being able
to target BU exposure has been illustrated in the above discussion of reports correlating Bu
pharmacokinetic parameters with its toxicity and efficacy. Thus, by utilizing a once daily
IV dose, targeted to prespecified levels, Bu therapy can be optimized to limit toxicity and
maximize efficacy.

Rituximab is an anti-CD20 human-mouse chimeric monoclonal antibody indicated for the
treatment of relapsed or refractory, low-grade or follicular, CD20-positive, B-cell NHL. The
use of rituximab, as monotherapy, results in significant responses and durations of response
in patients with indolent NHL; its efficacy improves even further when used in combination
with chemotherapy for both indolent and aggressive NHL. Responses to rituximab treatment
vary by the histologic subtype of NHL. In patients with low-grade follicular lymphoma,
response rates are approximately 70% to 75% with first-line therapy. On the other hand,
patients with diffuse B-cell large-cell lymphoma (DBLCL), single-agent rituximab results in
response rates of 30% to 40% have been reported. Rituximab significantly increases response
rates when used in combination with cyclophosphamide, doxorubicin, vincristine, and
prednisone.

CD20 is not expressed by hematopoietic stem cells, and therefore, we hypothesize that
treatment with rituximab is safe and would not inhibit engraftment produced by these early
progenitors. Treatment with rituximab may successfully eliminate minimal residual disease,
further delaying or preventing disease relapse and potentially extending the duration of
survival in CD20+ expressing malignancies.