For CLL Cells, There’s No Place Like Home

An important distinction in oncology is the division between liquid tumors, in which the mobile nature of neoplastic cells is a fundamental aspect of their identity from the time of cancer initiation, and solid tumors, in which the neoplastic cells generally do not acquire the capability for widespread dissemination until later in the disease course. An interesting study by Andritsos et al. in this issue of Leukemia and Lymphoma postulates that this inherent mobility of chronic lymphocytic leukemia (CLL) cells may be exploited as a component of anti-tumor therapy [1]. The trial is based upon the assumption that shifting CLL cells from the peripheral tissues (i.e. lymph nodes, spleen, and bone marrow) into the circulation may sensitize the cells to other anti-neoplastic agents. To determine whether this hypothesis is correct, the feasibility and safety of shifting CLL cells between tissue compartments needs to be established. The study by Andritsos et al. demonstrates that plerixafor, a CXCR4 antagonist, can be safely used in patients to move CLL cells from secondary lymphoid organs into the peripheral blood, and thus this trial represents an early step towards exploiting cancer cell mobility as a therapeutic Achilles heel.

On a molecular level, the chemokine receptors C-X-C motif receptor 5 (CXCR5) and CXCR4 play a critical role in physiologic lymphocyte trafficking to the lymph node as well as pathophysiologic recruitment of CLL cells to the protective stromal microenvironment [2-4]. CLL cells circulating in the blood are recruited to secondary lymphoid organs as they follow a chemotactic gradient of CXCL12 and CXCL13 (ligands for CXCR4 and CXCR5, respectively) secreted by stromal cells within the organs. Once in this protective niche, additional signals from the microenvironment contribute to CLL cell survival (reviewed in [5]). Thus, interruption of homing to the lymphoid organs may drive the cells into the circulation, where they are deprived of these survival signals and could potentially be more susceptible to therapy.

The observation that pharmacologic agents can shift CLL cells between compartments in the body dates to at least the 1940s and 1950s, when glucocorticoids were noted to cause a transient lymphocytosis in patients with CLL; this increase was postulated to be due to redistribution of cells from the peripheral tissues into the circulating blood [6]. This class of agents is also directly toxic to CLL cells, making it difficult to differentiate the therapeutic effects of mobilization from direct cytotoxicity. More recently, the BTK inhibitor ibrutinib [7] and the PI3K inhibitor idelalisib [8] have both been shown to cause a redistribution lymphocytosis in the setting of substantial disease reduction. In a murine model of CLL, ibrutinib decreases surface levels of CXCR4 while reducing homing of leukemic cells to the spleen [9]. Additionally, ibrutinib decreases signaling downstream of CXCR4 [9] and other cell adhesion molecules such as integrins [10] in vitro. Thus, multiple mechanisms may explain the redistribution lymphocytosis seen with BCR inhibitors. The degree to which the mobilization of CLL cells contributes to the effectiveness of BCR inhibitors, as opposed to direct effects on cell proliferation and cell death, remains under investigation [11]. However, mobilization is unlikely to be the predominant mechanism of efficacy, as multiple other BCR inhibitors with much less disease efficacy have nonetheless been able to cause substantial mobilization in early phase clinical trials [12-15].

Andritsos et al. performed a phase 1 trial examining the use of plerixafor in combination with rituximab in patients with relapsed/refractory CLL. By using the CXCR4 antagonist, the authors were able to examine the safety and efficacy of CLL cell mobilization strictly independent of any direct cytotoxic effects, which are lacking with plerixafor. A subcutaneous dose of 0.32mg/kg plerixafor resulted in a 4.4-fold increase in circulating CLL cells and avoided nausea and hypersensitivity reactions that limited the use of a higher dose. This finding validates CXCR4 as an important receptor in human CLL biology and demonstrates in vivo that interruption of CXCR4 signaling mobilizes cells from the secondary lymphoid organs into the blood, a concept which had previously been tested only preclinically [16]. It is interesting that the authors do not observe a dose dependence of CLL mobilization, perhaps suggesting a maximal effect at lower doses and/or a cap on the capacity to mobilize cells through blockade of this pathway alone. It is unclear how much of a lymphocyte shift occurred, i.e. on a per-patient basis what percentage of disease remained within protective niches (and perhaps less susceptible to rituximab). The mobilization effect of plerixafor also waned over time; a median 1.5-fold increase was seen with the day 26 dose compared to a 4.4-fold increase with the day 8 dose, despite similar pharmacokinetic profiles each day. This difference may represent a lower burden of cells in secondary lymphoid organs after time on therapy or may reflect a rapid adaptation of CLL cells to inhibition of CXCR4 signaling.

The ability to mobilize CLL cells independent of any therapeutic effect naturally leads to the question of whether mobilization can make these cells more susceptible to other therapies. Here, plerixafor was combined with single agent rituximab, a drug which has response rates of roughly 35–45% [17,18] when given at the high doses in this setting. Definitive conclusions about efficacy are limited in this single-arm phase I study of 23 patients. The patient population was relatively young, with early stage disease and favorable cytogenetics (40% 13q deletion and 0% 17p deletion by Döhner hierarchy). This population does not reflect the ibrutinib-refractory nature of the patients with relapsed CLL typically seen today -- given the known effects of ibrutinib on cytokine signaling, this is likely an important difference. Nevertheless, the response rate of 38% reported here with plerixafor plus rituximab is comparable to historical data for rituximab monotherapy in similar patient populations.

While plerixafor does not have a major effect on efficacy that can be detected in a small phase I trial, multiple questions remain for further study. For example, the degree of CLL cell mobilization with plerixafor may be less than with ibrutinib, which inhibits CXCR4 and other signaling pathways through multiple mechanisms. The potential for synergy of ibrutinib and a CXCR4-based therapy would therefore depend on whether ibrutinib induced redistribution is already maximal, which is not known. From the standpoint of potent clearing of mobilized cells from peripheral blood, it is not clear that rituximab is the best agent to combine with a CLL cell mobilizer, whether a CXCR4-based therapy or ibrutinib, particularly as two randomized trials have shown that adding rituximab to ibrutinib does not extend progression free survival compared to ibrutinib alone [19,20]. A more potent anti-CD20 antibody such as obinutuzumab would be preferred, but perhaps the best partner would be venetoclax, with its very high efficacy in clearing cells that are mobilized into the blood. A CLL cell mobilizer might be particularly useful in eradicating minimal residual disease in combination with a drug like venetoclax, rather than being needed during an initial debulking phase. While many interesting questions remain, ultimately, the trial by Andritsos et al. highlights the potential for exploiting cancer cell mobility to escape the microenvironment and increase vulnerability to effective therapy, a principle likely to be important as we move into an era of combination therapies for CLL.