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The measurement of response to anticancer therapy has evolved over the years as a result of better therapies and progress in imaging.
Oncology & Biotech News
Chairman and Director Lymphoma Division Chief John Theurer Cancer Center at HackensackUMC Chief Science Officer and Director of Research and Innovation Regional Cancer Care Associates Professor of Medicine, Georgetown University
The measurement of response to anticancer therapy has evolved over the years as a result of better therapies and progress in imaging. Minimal residual disease (MRD) refers to the small number of malignant cells that remain after therapy when the patient is in remission and shows no symptoms or overt signs of disease. Because MRD is seen as the major cause of disease relapse, it has been a goal of oncologists for a number of years to reach an MRD-negative status; unfortunately, this has not been translatable in routine use and therefore occurs mostly in larger institutions and/or on clinical trials.
Achieving a complete response (CR) in oncology translates into a better outcome. The measurement of response to therapy has improved greatly through imaging technology, initially with CT and later with functional imaging. Although the specific criteria differ between solid and liquid tumors, response has been the reference point for patient management, clinical trials, and drug development, with adjustment over the years. In lymphoma, for example, response criteria have been updated no fewer than three times since the mid 1990s,1 with refined parameters such as allowing a residual mass to be called a CR as long as the PET scan is negative. However, in liquid tumors, other sites of disease are important, particularly blood and bone marrow. Flow cytometry allowed—for the first time—the quantification of tumor cells beyond morphology evaluation under the microscope. The more advanced multicolor flow cytometry can reach sensitivity close to molecular biology—based approaches and can be particularly useful in leukemia, though this requires great technical expertise and is still mainly used in the context of clinical trials.2
A number of molecular biology techniques have been tested over the years for diagnostic, prognostic, and MRD purposes, including cytogenetics, fluorescence in-situ hybridization (FISH), comparative genome hybridization (CGH), and Southern blotting, though these are more cumbersome and usually lack the sensitivity required for MRD monitoring. The markers used for DNA-based testing are often chromosomal translocations, such as t(8;21) and t(15;17) in acute myeloid leukemia (AML/acute promyelocytic leukemia [APL]) or t(14;18) and t(11;14) in lymphoma, as examples.
The most commonly used technique for MRD is the allele- specific oligonucleotide PCR (ASO-PCR) based on the fact that both B- and T-cell leukemias or lymphomas exhibit a distinct immunoglobulin (BCR) and TCR gene rearrangement that is specific to a given clone. However, this requires the development of reagents (patient-specific probes) and assay conditions for each individual patient, which is laborious, expensive and time-consuming. On the other hand, mRNA-based tests are used when a DNA test is impractical and though handling of RNA is more difficult (less stable), these translocations present the advantage of not being specific to an individual patient and remain stable throughout the course of the disease. Examples of RT-PCR used include t(9;22) BCR-ABL in chronic myeloid leukemia (CML), t(15;17) PML-RARA in APL, and t(12;21) ETV6-RUNX1 (TEL-AML1) in AML.
Using real-time quantitative PCR (RQ-PCR), investigators in Europe were able to define international standards and obtain reproducible large-scale data sets as part of a multi-center global trials setting that has basically set the gold standard for MRD studies, with sensitivity overall in the range of 10-4 to 10-5.3 Such efforts, which should be commended, confirmed that obtaining a CR, particularly an early molecular CR in mantle cell lymphoma (MCL), had a profound impact on outcome, including overall survival. The more recent development of next-generation sequencing has the potential to transform MRD assessment. Without going into too much detail, this approach allows for quantification of allelic burden by counting the number of wild-type sequences versus mutated sequences (with no need for a sequence-specific probe).4
Determining treatment efficiency: MRD as a prognostic factor of relapse
A number of models (prognostic index, biomarkers) have been developed over time to predict the outcome of a given patient based on baseline clinical presentation. However, these models can be affected by the evolution of therapies (eg, rituximab’s effect on IPI in lymphoma), or are routinely trumped by newer molecular signatures. The best prediction of outcome in any patient with cancer is the actual response to therapy.
Due to its a posteriori nature, MRD detection is probably less susceptible to treatment-related variability than biomarkers and, therefore, is likely a better marker of quality of response and predictor of relapse. For example, in CML, where tyrosine kinase inhibitors (TKIs) have transformed the treatment paradigm, responses have shifted from hematological to cytogenetic and now molecular CR, which translates into superior outcomes and defines new endpoints. Similarly, achieving a molecular CR in MCL and chronic lymphocytic leukemia (CLL) post-induction translates into improved outcomes.
More recently, emerging data based on assessment of circulating tumor cells in large cell lymphoma suggests that most patients have detectable circulating tumor cells or clonal naked DNA at baseline. In addition, in small series so far, achieving a molecular CR in peripheral blood lymphocytes (PBL) was feasible after two cycles of conventional chemoimmunotherapy and translated into an excellent outcome, regardless of initial presentation. This might become a new endpoint in large cell lymphoma, pending validation in ongoing studies.
MRD as remission control and trigger for pre-emptive treatment
The treatment of relapsing leukemias or lymphomas is associated with high mortality/morbidity and usually leads to a much worse outcome. The idea of early detection of molecular relapse has been used to trigger early intervention. Though not necessarily a definitive answer (ie, leading to a cure), such strategies have clearly led to an improvement in progression-free survival and disease control. For example, in MCL, additional rituximab was given to patients who had a molecular relapse after high-dose therapy, leading to conversion to negative PCR again in most patients.5
Other examples of intervention include cell therapy with donor lymphocyte infusion in molecular relapses following allogenic stem cell transplantation. One of the most critical impacts of monitoring MRD has been in CML patients receiving TKIs, where it has allowed the detection of early resistance and switching therapy earlier.
This has not only been shown to improve patient outcomes, but is also likely to reduce costs through an earlier intervention overall (ie, PCR monitoring leads to request earlier resistance testing by opposition to risk of transformation and need for aggressive measures, including allogenic BMT).
A number of novel therapies are very promising in CLL, such as drugs targeting the BCR pathway, including ibrutinib and idelalisib. Though the response rates with these new drugs are very high—in the 70% range and above— most patients achieve only a partial response. However, combining these agents with monoclonal antibodies or integrating them as part of other therapies might allow deeper responses, particularly in light of their ability to “demobilize” tumor cells from their microenvironment. This provides an attractive rationale for a number of ongoing studies, particularly in MCL and CLL.
MRD instead of imaging for surveillance after therapy?
The recent observation of MRD detection in aggressive lymphomas after induction therapy, with MRD (PBL) found positive in most cases before any CT findings, suggest this measure could replace imaging in the future. Both sensitivity and specificity were very high in these early small series. Though this needs to be validated prospectively, this could provide savings both in cost and reduced imaging exposure, with obvious and appealing benefits.
MRD has evolved from a “dream goal” to a likely achievable goal in a number of diseases and should be pursued actively, both in clinical trials and in routine practice, where it has the potential to become a new endpoint— a new benchmark that can hopefully help improve outcomes for our patients.