A previous study of the interaction between Pgp and regimens with or without fludarabine (comprising fludarabine, AraC, idarubicin±etoposide) adopted an age-matched case–control strategy and found an improved CR rate, OS and disease-free survival for Pgp-positive patients with regimens containing fludarabine.16
In the randomised AML15 trial, results on CR are in agreement with this and show a CR/CRi rate of 86% for Pgp-positive patients with FLAG-Ida, which was the same as that seen in Pgp-negative AML. In contrast, patients receiving induction regimens incorporating the Pgp substrates daunorubicin and etoposide (DA/ADE) had a lower CR rate if they were Pgp+. This is the second randomised controlled trial in de novo
AML to have shown a benefit for an intervention in the Pgp+ group. Previously, quinine was also shown to improve the CR rate but not OS in Pgp+ AML patients.30
Other strategies attempted, particularly the use of Pgp inhibition with PSC833 or zosuquidar, have failed to improve outcomes in this group of patients.7, 8, 9, 10, 11, 12
Regarding relapse, results for AML15 previously reported showed that FLAG-Ida resulted in a reduced relapse rate overall compared with DA/ADE.29
Both Pgp-positive and -negative patients benefited in this respect from FLAG-Ida treatment, but in contrast to the case–control study, we found a significant interaction between Pgp and treatment received with respect to relapse rate, with Pgp-positive patients having a higher relapse rate than Pgp-negative patients in both treatment arms. Furthermore there was no interaction between Pgp and treatment received in effect on OS. Thus the use of FLAG-Ida induction in Pgp-positive cases induces a slight shift in initial efficacy, such that efficient clonal reduction results in improved CR for this subgroup, but these patients have an increased frequency of relapse with a consequent lack of effect on OS. These data suggest that in the most chemoresistant samples, the leukaemic clone is reduced rather than eradicated, such that relapse is high in these cases. Pgp is not necessarily overexpressed in all leukaemic stem and progenitor cells. However, we have previously determined that Pgp expression in the CD34+CD38 subset, which contains the leukaemia initiating cells, correlates strongly with expression in the sample as a whole, suggesting that when a sample is Pgp positive, relapse is likely to be driven by Pgp-positive cells within this sub-population.31
However, factors driving chemoresistance in the MRD cells of Pgp-negative samples remain to be determined. The suggestion that Pgp is acquired during the course of remission-induction chemotherapy is a possibility, but remains unproven.11, 32, 33
We investigated an alternative explanation for the altered CR and relapse rates in Pgp+ patients treated with fludarabine-based regimens, namely that post-remission regimens, including use of stem cell transplants, may have differed. Consolidation regimens were randomised in the AML15 trial, although uptake in the FLAG-Ida arm of the trial was lower than in the ADE/DA part of the trial. Overall within AML15, as was demonstrated in the initial report of the results,29
significantly improved relapse-free survival was obtained with FLAG-Ida compared with daunorubicin-containing regimens. In the study reported here, rates of allogeneic transplantation were similar between groups (ADE/DA 20%, FLAG-Ida 15%, P
=0.2). We therefore consider it unlikely that the results seen arise out of differences in post-remission treatment, although numbers are too small to rule this out with absolute certainty.
Slight distinctions between Pgp protein and function have been noted in this study and we have commented on this previously.1
We used PSC-833 in our functional assay and although this is more specific than other modulators in the literature, it has not been studied in genetic models of all 49 ABC transporters, and other efflux pumps may be affected in this assay. The major impact of functionally defined Pgp in our study was on relapse, so the possibility that the leukaemic stem cells that drive relapse have additional pumps affected in the functional assay is logical but completely speculative.
Our study shows a high CR/CRi rate in AML15 Pgp-positive patients compared with other studies reported in the literature. represents a compilation of remission data, from studies in which the Pgp status of patients is known. Some studies comprised a wide range of patient ages, so we have selected those with a median age <60 to afford a reasonable comparison with our data. All patients were treated with chemotherapy containing ara-C and an anthracycline. The CR/CRi rate for Pgp-positive adult patients in AML15 receiving FLAG-Ida was over 10% higher than that reported in these historical controls.
CR/CRi rates for Pgp-positive and -negative previously untreated patients with a median age <60 (listed in order of increasing median age)
Only one report of those listed in failed to show a difference in CR rate between Pgp-positive and -negative patients: this study is notable in that idarubicin was used in preference to daunorubicin, and fludarabine was not included in the treatment protocol.19
This serves as a reminder that the effects we have observed in the current study maybe a result of the interaction between fludarabine and idarubicin or to either of these drugs individually. Prospective analysis of Pgp in clinical specimens has been hampered by a literature in the 1990s showing that Pgp is difficult to measure. More recently, however, multi-centre analysis of AML samples has shown that reliable, quick measurements of Pgp function are feasible.1, 26
We hope that other groups using fludarabine and idarubicin will consider measuring Pgp in their patients.
In conclusion our data indicate that FLAG-Ida may improve the remission rate for Pgp-positive AML, but the malignant clone is reduced rather than eradicated such that the relapse rate remains high in Pgp-positive patients, irrespective of remission-induction therapy. However the high CR rate obtained with FLAG-Ida in Pgp-positive AML may permit more patients to progress to allogeneic transplantation in first CR.