Drug Sensitivity by TRAC (DiSC) Assay as a Prognostic Factor for Patient Response in Untreated CLL: Results from the UK LRF CLL4 Trial.
Authors: Bosanquet Andrew G. , Raper Steve L. , Durant Justin , Scadding Sheila M. , Oscier David Graham , Richards Sue M. , Catovsky Daniel ,   on Behalf of the NCRI CLL Working Group
Published: October 2019

Background. The UK LRF CLL4 trial opened in 1999: 777 previously untreated patients have been randomised between chlorambucil (Chl), fludarabine (Flu) and cyclophosphamide (Cy) plus Flu (FluCy).

Methods. We measured Drug Sensitivity prior to initial therapy by TRAC (tumour response to anti-neoplastic compounds) assay, a development of the DiSC assay and divided patients (pts) into three groups: Drug resistant (DR, LC90 >= 10 ug/ml for Flu and mafosfamide (for Cy), >6.3ug/ml for Chl), Drug sensitive (DS, <0.87ug.ml Flu, <2.3ug/ml Chl, <3.3ug/ml mafosfamide) and Intermediate (DI, others).

Results. Response rates by TRAC (Table 1) show a good correlation with clinical response. For example, 49% of all pts were DS and 90% of them responded to chemotherapy; whilst 9.5% were DR and only 13/42 (31%) responded. Of the 24 DR to Chl, 17/24 (71%) were DS or DI to Flu and 100% were DS or DI to FluCy. Of 14 pts DR to Flu, only 5 (36%) were DS or DI to FluCy.

Table 1. Response rates compared with Drug Sensitivity to drug(s) received Drug Sensitivity Chl Flu FluCy Overall Data are: Response rate (%) (# pts) DS 85.1 (94) 90.7 (54) 95.7 (70) 89.9 (218) DI 66.3 (92) 79.2 (53) 97.3 (37) 76.4 (182) DR 37.6 (24) 21.4 (14) 25.0 (4) 31.0 (42) Total 71.5 (210) 77.7 (121) 93.7 (111) 78.7 (442)

Biological markers with prognostic significance were also measured at trial entry (Oscier et al, ASH 2006). Whilst pts with mutated (mut) VH genes had above average response, p53 loss (>20%) was associated with in vivo drug resistance and very poor outcome. Thus, pts could be classified into three groups with Low, Standard or High risk of disease progression based on VH genes and FISH (Table 2).

Table 2. Comparison of Drug Sensitivity with Risk Group Risk Group Drug Sensitivity Low (mut VH) Standard (unmut VH, VH 3–21, del11q) High (p53 loss) Overall DS 90.7 (75 90.1 (111) 66.7 (3) 89.9 (189) DI 77.1 (48) 75.2 (105) 33.3 (6) 74.2 (159) DR 75.0 (4) 30.0 (20) 21.4 (14) 31.6 (38) Overall 85.0 (127) 78.3 (236) 30.4 (23) 77.7 (386)

While almost all patients without p53 loss respond to FluCy, 30% do not respond to Chl and only a minority of these have p53 loss. The potential importance of TRAC is its ability to identify drug resistant pts and to predict sensitivity to alternative treatments for these, as well as for pts with p53 loss. 47 of 386 pts (12%) were either DR by TRAC to treatment given or were High Risk with p53 loss: 45 (96%) were DS or DI to a variety of CLL regimens including Chl, Flu, FluCy, high-dose methylprednisolone, COP (Cy + vincristine + prednisolone), pentostatin and anthracyclines.

Conclusions. TRAC assays are effective at identifying pts with a low or high probability of response. Furthermore, a TRAC result gives indications of which treatment regimes are more likely to work in the proportion of pts with p53 loss or who are resistant to standard first-line chemotherapy regimens. A second randomisation in the CLL4 trial is designed to establish whether, after relapse or non-response to initial therapy, treatment selection based on a TRAC assay improves response and survival.