RHPS4, An Anti-Telomere Agent, Specifically Targets the Subgroup of AML Cells with Loss or Gain of Parts of Chromosomes.

Abstract 1195

Cytogenetic abnormalities vary greatly in AML and those with gains and losses of chromosome parts, terminal deletions and whole chromosome losses could theoretically derive from telomere dysfunction. Cells with these abnormalities are characterised by telomere shortening and abnormal function characterised by activation of both the negative telomere regulator TRF1 and also of hTERT. The G quadruplex ligand RHPS4 (Pharminox Ltd, Biocity, Nottingham, UK) binds telomeres and induces DNA damage at higher doses in the short term and critical telomere shortening in longer term growth assays. We have previously reported cytostatic and cytotoxic effects for this agent in AML cell lines, whilst others have found that normal haematopoietic progenitor cells are not targeted. We have now examined effects of RHPS4 on the in vitro growth and survival of primary AML cells, comprising 9 samples with normal and 8 with abnormal karyotypes (5 complex, one -7, one 5q- and one with additional material on chromosomes 2 and 9). Cells were incubated with RHPS4 in standard 14 day colony assays and for 7 and 21 days in a novel suspension culture medium supplemented with supernatant from cultured normal human osteoblasts. In the 7 day cultures, no effects were observed on cell survival. In the longer term growth assays, a submicromolar IC₅₀ for RHPS4 was observed in 7/8 samples with aberrant cytogenetics in contrast with only 1/9 with normal cytogenetics (P=0.003, Fisher's exact test). Furthermore, RHPS4 significantly sensitised samples with abnormal cytogenetics to daunorubicin reducing the median IC50 from 27nM to 10nM (n=8, p<0.05).

A qPCR method was used to measure telomere length of untreated DNA from 15 of the samples; the qPCR assay had previously been performed in a separate patient cohort alongside a Southern technique and comparable values were attained for the two different methods. There were no significant differences in telomere lengths between the normal and abnormal cytogenetic groups suggestive that telomere length is not a determinant of RHPS4 sensitivity in these assays. Samples with adverse cytogenetics have previously been associated with an up-regulation of basal DNA damage markers and may therefore be sensitive to compounds that can exacerbate levels of damage. We therefore studied the levels of p-ATM by immunocytochemistry in RHPS4-treated cells and found a significant increase compared to untreated cells - hence RHPS4 may be acting via a DNA damage mechanism.

We conclude that telomere targeting has exciting potential as therapy for a subset of AML patients including those with adverse cytogenetics who have a poor response to conventional chemotherapeutics.