Pharmacodynamics of cytarabine induced leucopenia: a retrospective cohort study

Abstract

AIMS

Cytarabine is a pyrimidine analogue used to treat a variety of haematological malignancies. There are few data regarding the pharmacodynamics of cytarabine. The only publications regarding this issue cite a biphasic pattern of decline in white blood cell (WBC) counts following low and intermediate doses, in patients with various malignancies, most of them non-haematological. Our purpose was to establish the pharmacodynamics of cytarabine induced leucopenia in acute myeloid leukaemia (AML) patients treated with contemporary cytarabine containing protocols.

METHODS

We conducted a retrospective cohort study, including 56 patients with AML in complete remission who had received 89 cycles of intermediate or high dose cytarabine. Daily counts for WBCs and neutrophils (ANC) were collected during the first 15 days after the initiation of cytarabine administration and pharmacodynamics were analyzed. Further analysis was carried out to correlate between WBC and ANC pharmacodynamics and different cytarabine protocols [high dose cytarabine (HiDAC) vs. intermediate dose cytarabine (IDAC)].

RESULTS

Analysis of blood counts demonstrated a monophasic decline of WBCs and ANCs, unlike a previous depiction of a biphasic pattern. HiDAC was associated with a significantly sharper decline of WBCs than IDAC.

CONCLUSIONS

Our data support a monophasic decline pattern of WBCs and ANCs following contemporary cytarabine protocols. The decline rate is steeper for patients receiving HiDAC than for those receiving IDAC. These results might help form evidence based guidelines regarding patient monitoring intensity, timing of prophylactic antibacterial and antifungal treatment as well as growth factors' support following cytarabine based consolidation for AML.

WHAT IS ALREADY KNOWN ABOUT THIS SUBJECT

• There are few data regarding the pharmacodynamics of cytarabine induced leucopenia.

• Lack of data leads to different institutional policies regarding the supportive care for acute myeloid leukemia (AML) patients.

• An updated analysis of cytarabine induced leucopenia might help form evidence based recommendations regarding supportive care post-cytarabine based consolidation for AML patients.

WHAT THIS STUDY ADDS

• This is the largest cohort describing the pharmacodynamics of leucopenia post-cytarabine consolidation therapy for AML.

• White blood cell (WBC) decline is monophasic and steeper for high dose cytarabine (HiDAC) than for intermediate dose cytarabine (IDAC).

• These results might contribute to the establishment of guidelines regarding patient monitoring intensity, antibacterial and antifungal prophylaxis and the administration of growth factors in AML.

Introduction

Cytosine arabinoside or cytarabine (Cytosar®) is a synthetic pyrimidine analogue originally isolated from the sponge Cryptothethya crypta [1] and now produced synthetically [2]. Cytarabine is commonly used to treat acute myeloid leukemia (AML) and is incorporated into virtually all standard induction (generally in combination with an anthracycline) and consolidation protocols for this disease [3] as well as for most salvage protocols. High dose cytarabine seems to be especially beneficial for core binding factor AML [4]. Cytarabine is also incorporated in protocols for other haematological malignancies such as acute lymphoblastic leukaemia (ALL) and lymphoma, either as primary treatment or for salvage [5,6].

Cytarabine acts by competitive inhibition of DNA polymerase, thus inhibiting both DNA synthesis and DNA repair [7–9]. In addition, cytarabine becomes incorporated into the DNA, a feature which correlates well with its cytotoxicity and is probably the most important mechanism of its efficacy [10–12].

Cytarabine penetrates cells by a carrier-mediated process shared by other physiologic nucleosides [13]. The most important transporter seems to be hENT1, which has rate limiting effects on cytarabine activity [13,14]. At drug concentrations higher than 10 μmol l^–1, achieved by infusing 1 to 1.5 g m^–2 over 3 h, the transport process becomes saturated and further entry takes place by passive diffusion [15–17].

The pharmacokinetics of cytarabine are characterized by rapid disappearance from plasma owing to deamination [18]. The steady-state concentration of cytarabine in plasma achieved by constant intravenous infusion remains proportional to dose for dose rates up to 2 g day^–1. At higher rates of infusion, the deamination reaction is saturated and cytarabine plasma concentrations rise unpredictably, which leads to severe toxicity in some patients [19,20].

Although cytarabine is a commonly used drug with potentially severe and life threatening side effects [21–24], its pharmacodynamics have not been well studied. The haematological effects of cytarabine on the patient can be quantified due to bone marrow suppression induced by its administration. Leucopenia is universal among patients with haematological malignancies treated with cytarabine as part of treatment protocols due to the high cumulative dose and prolonged exposure. However, published data regarding the pharmacodynamics of cytarabine induced leucopenia are scarce. Several databases cite [25–27] the ECOG coordinated, NIH sponsored work of Burke et al. from the late 1960s, who described a biphasic pattern of cytarabine induced leucopenia, with a nadir on days 7–9, a brief recovery on days 10–13 and a second nadir between days 15–24 post-treatment [28,29]. This pattern was described after the administration of low and intermediate doses of cytarabine (50–600 mg m^–2) for patients with a variety of malignancies, most of them non-haematological. Since then no study evaluating the pharmacodynamics of cytarabine has been published.

Since cytarabine is a cornerstone in the treatment of haematological malignancies and since understanding its pharmacodynamics might contribute to the development of clinical recommendations in these patients, we decided to examine the pharmacodynamics of cytarabine in a large, homogenous cohort in our institute.

Methods

This was a retrospective cohort study. The study was approved by the IRB committee of our institution. The study population included all AML patients older than 18 years in complete remission admitted to our ward between February 2007 and February 2013 for consolidation treatment with intermediate dose cytarabine (IDAC) or high dose cytarabine (HiDAC). In our institute HiDAC includes six doses of cytarabine at 3 g m^–2 twice daily on days 1, 3 and 5. IDAC contains 12 doses of cytarabine at 500 mg m^–2 twice daily for 6 consecutive days. Daily complete blood counts including data for WBCs, ANCs, lymphocytes, monocytes, platelet counts and haemoglobin were collected during the first 15 days after the initiation of cytarabine administration and their kinetics were analyzed. According to our routine practice blood samples were drawn daily around 06.00 h after a night's fast. Samples were processed by the ADIVA 2120/2120i Haematology System analyzer (Siemens Healthcare Diagnostics, Deerfield, Illinois, USA) within 4 h following phlebotomy. Differential analyses with reporting of ANC counts were carried out using the peroxidase cytochemical staining on all samples. The quality control programme included harmonization procedures and three level (high, low and medium) internal quality controls (R&D Systems, Minneapolis, Minnesota, USA), as well as a monthly external quality control (UK NEQAS) assessment which showed a mean bias of 0% for WBCs (1% for RBCs, 1.7% for platelets).

We used anova with repeated measures to assess changes in blood counts over time in order to establish the pharmacodynamics of cytarabine induced leucopenia.

A linear descent of WBC and ANC will be correlated with time, while a biphasic descent will correlate with either time squared or time cubed, depending on the steepness of the biphasic part. Since a biphasic pattern was the only one described for cytarabine pharmacodynamics so far, we assessed the possibility of this pattern by adding time squared and cubed time to the model. We performed further analysis to assess possible associations of changes in blood counts with cytarabine protocols (IDAC vs. HiDAC) and addressed the issue of possible confounding factors such as steroid or granulocyte colony stimulating factor (G-CSF) treatment, renal failure or active infections. As a double nadir could appear at different time points for different patients, thus escaping detection with the above mentioned method, we plotted the total WBC and ANC according to time from cytarabine administration for every single patient and asked a physician unaware of the study's design and purpose (TD) to mark nadir points. The number of plots with a double nadir was noted.

Results

The study population included 56 patients with AML in complete remission who received 89 cycles of cytarabine (Table 1). There were 29 males and 27 females. Average age was 55, range 18–79 years. Seven patients had favourable risk AML, 42 had intermediate risk and seven had poor risk disease according to karyotype and genetic markers [30]. Thirty-five patients received one course of cytarabine, nine patients received two courses and 12 patients received three courses. Thirty-two patients were treated with HiDAC and 24 with IDAC. Three patients were treated with corticosteroids during consolidation and five with G-CSF. Three patients had major infections during the first 2 weeks of chemotherapy. Twelve patients had renal impairment, defined as glomerular filtration rate lower than 60 ml min^–1 at some point during cytarabine administration.

Table 1.

Patient characteristics

Number of patients
56	Total
27 (48%)	Female gender
7 (12.5%)	Favourable risk cytogenetics*
42 (75%)	Intermediate risk cytogenetics*
7 (12.5%)	Poor risk cytogenetics*
32 (57%)	HiDAC protocol
24 (43%)	IDAC protocol
3 (5%)	Corticosteroid administration during the first 14 days of therapy
5 (9%)	G-CSF administration during the first 14 days of therapy
3 (5%)	Major infections during the first 14 days of therapy
12 (21%)	GFR<60 ml min−1
35 (62.5%)	Received 1 cycle of cytarabine
9 (16%)	Received 2 cycles of cytarabine
12 (21.5%)	Received 3 cycles of cytarabine

^*According to [32].

Repeated anova measures of blood counts with time, time squared and cubed time revealed a significant effect only for time (P<0.0001 for time, P=0.4931 for squared time and P=0.8035 for cubed time). Thus, analysis of blood counts was compatible for a monophasic decline for WBCs and ANCs with no biphasic component. WBC and ANC plots depicted for individual patients did not show a biphasic pattern. The physician unaware of the study's design and purpose (TD) marked only eight out of 178 (4.5%) plots as compatible with a biphasic pattern. No effect of possible confounders such as corticosteroids or G-CSF treatment, renal impairment, active infections or karyotype was observed, possibly due to the low number of events suspected as potential confounders (Table 1). No difference between first, second or third cycle pharmacodynamics was found. The pattern was similar for HiDAC and for IDAC therapy. However, the treatment protocols used were significantly different for the slope of the descent of the blood counts: WBC and ANC counts of patients treated with HiDAC fell faster than those of patients treated with IDAC (P = 0.007 for WBC and P = 0.003 for ANC) (Figures 1 and 2). On average, patients treated with HiDAC developed severe neutropenia (less than 0.5 × 10⁹ l^–1) on day 10 post high dose cytarabine administration initiation, while patients treated with IDAC achieved it on day 14. Figures 1 and 2 depict the descent of WBC and ANC medians post HiDAC and IDAC treatment. Table 2 shows the daily median, mean, standard deviation and confidence interval of WBCs and ANCs post-HiDAC and IDAC treatment.

Figure 1.

Leucocyte count (× 10⁹ l^–1) after HiDAC and after IDAC administration according to days post-initiation of treatment. Each point represents the median leucocyte count on a given day. , IDAC; , HiDAC

Figure 2.

Absolute neutrophil count (× 10⁹ l^–1) after HiDAC and after IDAC administration according to days post-initiation of treatment. Each point represents the median neutrophil count on a given day. , IDAC; , HiDAC

Table 2.

Median and mean daily WBC and ANC counts following HiDAC and IDAC treatment

Protocol	Day	Cell type	Median cell count	Mean cell count	Standard deviation (SD) for mean	95% confidence interval (CI) for mean
HiDAC	1	WBC	5.710	6.385	3.252	5.479, 7.290
		ANC	4.286	5.003	3.077	4.146, 5.860
HiDAC	2	WBC	4.485	5.014	2.372	4.354, 5.675
		ANC	3.786	4.282	2.244	3.657, 4.907
HiDAC	3	WBC	3.425	4.144	2.002	3.597, 4.690
		ANC	3.001	3.570	1.955	3.036, 4.104
HiDAC	4	WBC	3.310	3.615	1.625	3.176, 4.055
		ANC	2.804	3186	1.581	2.759, 3.614
HiDAC	5	WBC	2.800	3.119	1.329	2.760, 3.478
		ANC	2.310	2.651	1.248	2.313, 2.988
HiDAC	6	WBC	2.810	2.902	1.141	2.587, 3.217
		ANC	2.459	2.539	1.055	2.248, 2.830
HiDAC	7	WBC	1.770	1.875	0.821	1.653, 2.097
		ANC	1.384	1.460	0.746	1.259, 1.662
HiDAC	8	WBC	1.380	1.449	0.722	1.252, 1.646
		ANC	0.9990	1.006	0.593	0.846, 1.1670
HiDAC	9	WBC	1.330	1.393	0.827	1.160, 1.626
		ANC	0.815	0.932	0.676	0.741, 1.122
HiDAC	10	WBC	1.030	1.098	0.789	0.880, 1.315
		ANC	0.477	0.6260	0.616	0.456, 0.796
HiDAC	11	WBC	0.640	0.758	0.483	0.627, 0.889
		ANC	0.183	0.2910	0.332	0.201, 0.381
HiDAC	12	WBC	0.620	0.694	0.459	0.570, 0.818
		ANC	0.1250	0.234	0.324	0.146, 0.321
HiDAC	13	WBC	0.640	0.676	0.391	0.571, 0.782
		ANC	0.0860	0.1900	0.240	0.125, 0.255
HiDAC	14	WBC	0.570	0.628	0.407	0.518, 0.738
		ANC	0.0680	0.163	0.313	0.078, 0.247
HiDAC	15	WBC	0.480	0.556	0.352	0.460, 0.651
		ANC	0.059	0.1120	0.155	0.070, 0.154
IDAC	1	WBC	5.455	5.298	1.927	4.626, 5.971
		ANC	3.763	3.760	1.773	3.141, 4.379
IDAC	2	WBC	4.585	5.169	2.365	4.344, 5.994
		ANC	3.631	4.156	2.373	3.328, 4.984
IDAC	3	WBC	4.115	4.352	1.463	3.842, 4.863
		ANC	3.143	3.599	1.484	3.081, 4.118
IDAC	4	WBC	3.645	3.908	1.400	3.419, 4.397
		ANC	3.085	3.246	1.433	2.746, 3.746
IDAC	5	WBC	3.605	3.621	1.221	3.195, 4.047
		ANC	3.023	3.049	1.235	2.618, 3.481
IDAC	6	WBC	3.395	3.204	1.249	2.768, 3.640
		ANC	2.654	2.701	1.224	2.273, 3.128
IDAC	7	WBC	2.865	2.678	1.065	2.307, 3.050
		ANC	2.065	2.076	1.060	1.706, 2.446
IDAC	8	WBC	2.470	2.487	1.125	2.088, 2.886
		ANC	1.798	1.964	1.093	1.576, 2.352
IDAC	9	WBC	2.235	2.428	1.317	1.968, 2.888
		ANC	1.682	1.874	1.268	1.431, 2.316
IDAC	10	WBC	2.145	2.001	1.184	1.588, 2.415
		ANC	1.393	1.455	1.082	1.077, 1.832
IDAC	11	WBC	1.740	1.761	1.203	1.341, 2.181
		ANC	0.956	1.173	1.045	0.809, 1.5380
IDAC	12	WBC	1.280	1.620	1.135	1.224, 2.016
		ANC	0.668	1.006	0.996	0.659, 1.354
IDAC	13	WBC	1.225	1.368	0.994	1.021, 1.715
		ANC	0.467	0.7500	0.817	0.465, 1.036
IDAC	14	WBC	0.815	1.193	1.168	0.785, 1.601
		ANC	0.2690	0.586	0.913	0.267, 0.905
IDAC	15	WBC	0.790	0.911	0.601	0.698, 1.124
		ANC	0.166	0.353	0.427	0.202, 0.505

ANC, Absolute neutrophil count; HiDAC, High dose cytarabine; IDAC, Intermediate dose cytarabine; WBC, White blood cells.

Discussion

Cytarabine is a cornerstone in the therapy of AML, being central to the most common induction, consolidation and salvage protocols [30,31]. Although cytarabine is widely used and its myelosuppressive effect can be easily quantified, not much is known about its pharmacodynamics. The only relevant data which can be obtained from both popular databases such as Micromedex or BC Cancer and from Pfizer's product monograph describe a biphasic, double-nadir effect on WBCs [25–27]. This is based on papers published by the ECOG coordinated, NIH sponsored work of Burke et al. 45 years ago, based on observations in patients with 15 different malignancies, mostly solid tumours [28,29]. Burke et al. treated patients for 5–10 consecutive days with daily doses ranging between 50–600 mg m^–2, with some patients treated with continuous infusions and others with intravenous boluses. These regimens differ substantially from the currently used IDAC and HiDAC protocols for AML.

To the best of our knowledge this is the only pharmacodynamic pattern described so far for cytarabine. Furthermore, this observation of a biphasic pattern has never been challenged in a large, homogenous cohort treated with contemporary protocols.

The study population in the present cohort included patients with AML who were treated with cytarabine as a single agent. Thus, it can serve as a reasonable model for its pharmacodynamics. Furthermore, we included only patients in complete remission in order to minimize potential confounding factors.

We could not show a biphasic pattern of leucopenia in our cohort. Thus, we believe that the double nadir phenomenon does not apply to AML patients treated with the contemporary cytarabine protocols. We found no effect of various possible confounding factors on blood counts. The only factor which had a clear effect on blood counts was the protocol used. Blood counts of patients treated with HiDAC descended more rapidly than those of patients treated with IDAC.

The difference between the post-cytarabine leucopenia pattern reported previously and our findings could have several explanations. Patient populations were different between our cohort and that described in the literature, AML vs. solid tumours. Although it is reasonable to think that this difference should not result in different pharmacodynamics of cytarabine induced leucopenia, the fact that in AML, unlike solid tumours, the disease involves the bone marrow itself might have an impact. Another likely explanation might be the different doses used. Lower doses of cytarabine such as those used by Burke et al. might induce a biphasic pattern of leucopenia, the first nadir through DNA polymerase inhibition and the second one through the late effect of DNA incorporation. Higher doses of cytarabine, as in our cohort, might cause a more linear descent in blood counts through greater enzyme inhibition or faster DNA incorporation with dose escalation, thus overcoming the ‘interrupting nadir effect’ and changing the pharmacodynamics described previously.

The difference in the slope of WBC and ANC descent following administration of HiDAC vs. IDAC could be explained by several mechanisms. Higher doses of cytarabine might inhibit DNA polymerase more than lower doses and might cause more DNA incorporation or saturation of the deamination process. Any one of these mechanisms, or a combination of several of them, may contribute to the faster descent of WBCs with HiDAC as compared towith IDAC. Of note, there was a considerable age difference between patients treated with IDAC (average age 66.7, range 49–79 years) and those treated with HiDAC (average age 46.6, range 18–61 years). This stems from the fact that as a general rule, in our institution, patients older than 60 years are treated with IDAC rather than HiDAC due to neurological toxicity. Bone marrow reserves are lower for the elderly population. Thus, if elderly patients were also included in the HiDAC arm the difference between the two slopes might have been even more pronounced.

WBC and ANC descent rate after cytarabine consolidation for AML patients might have clinical implications. There is no consensus between published guidelines regarding the issue of patient hospitalization post-consolidation treatment in AML [32,33].The result is that decisions regarding this issue are made based on the personal experience and preferences of the treating physicians and institutions. Furthermore, although there are suggestions regarding the use of growth factors post-consolidation treatment for AML, there is inconsistency regarding this issue as well [34,35]. The use of prophylactic antibiotics and antifungals in neutropenic patients has been of interest in recent years and is also under debate regarding the relevant population and time of initiation of prophylaxis [36–38]. Likewise, the choice of broad spectrum oral or parenteral antibiotics in febrile neutropenia is influenced by its depth and length, i.e. high risk vs. low risk neutropenia and is also a matter of debate [36].

Clinical decisions on these issues and the development of guidelines might be supported by knowledge of the pharmacodynamics of WBCs and neutrophils post-cytarabine based consolidation treatment. Data regarding descent rates might help in establishing a policy of patient monitoring and hospitalization, supportive care practices (i.e antibiotics and antifungal prophylaxis) as well as febrile neutropenia treatment. Differences between the pharmacodynamics of HiDAC and IDAC induced leucopenia might help in personalizing these issues to younger compared with older patients. Furthermore, when novel based therapies are added to the cytarabine backbone, knowledge of toxicity profile and dynamics might be of help.

Our study has several limitations. The cohort of patients was not large enough to calculate the true effect of the various confounders. The study was of a retrospective nature, which might have caused bias due to unknown or unrecorded confounders. However, as the study group included all suitable patients in a 6 year period in our institution we hope that this limitation was minimized. Due to the retrospective nature of our study we could not obtain cytarabine serum concentrations, cytarabine triphosphate concentrations in leucocytes and quantification of cytarabine incorporation into leucocyte DNA which might contribute to our understanding the pharmacodynamics of this important pyrimidine analogue. As this was a single centre study it was more vulnerable to unknown bias, but also helped make the study population more homogenous and prevented inter-laboratory differences.

In conclusion, we describe a monophasic pattern of descent of WBC after cytarabine treatment for AML patients. We found a difference in the kinetics of WBC and ANC descent between HiDAC and IDAC. Our results might be of interest for health professionals caring for patients with AML and might contribute to decision making regarding hospitalization, isolation and prophylactic antibiotic issues.

Competing Interests

All authors have completed the Unified Competing Interest form at http://www.icmje.org/coi_disclosure.pdf (available on request from the corresponding author) and declare no support from any organization for the submitted work, no financial relationships with any organizations that might have an interest in the submitted work in the previous 3 years and no other relationships or activities that could appear to have influenced the submitted work.