Look to history for a sense of the importance of today’s medical advances.
Most authorities recognize TR Ashworth, writing in the Australian Medical Journal, as making the first discovery of cells in the blood similar to those in patients’ solid tumors (post-mortem). That was in 1869. I couldn’t find the original article online, but the citation is: [Ashworth TR (1869) A case of cancer in which cells similar to those in the tumors were seen in the blood after death. Aus Med J 14: 146–149, as referenced in this minireview.]
Surely, shortly after word of this discovery spread, scientists began pondering ways of exploiting this knowledge to diagnose and track the progression of cancer. But a huge technological gulf separated the idea of using circulating tumor cells for the benefit of cancer patients and its application in the clinic. As you might imagine, the number of circulating tumor cells [CTCs] relative to the total number of circulating blood cells is relatively small, necessitating reliable tumor cell-enrichment and detection technologies to make diagnostic/theranostic use of CTCs.
Fast forward some 135 years from Ashworth’s initial discovery to the routine use of techniques to exploit it. In January 2004, Veridex, a J&J (JNJ) company, received US FDA approval to market CellSearch, a device for magnetically labeling antibody-selected epithelial cells in a blood sample, for use in patients with metastatic breast cancer as an aid to monitoring response to treatment. The theory behind the device and accompanying analysis of the BRCA CTCs is that the number of CTCs correlates (inversely) with response to treatment. To my knowledge, this is the only such separation/analysis system available in the U.S. for non-experimental monitoring of response to therapy via CTCs. It has now been approved for marketing for metastatic colon and prostate cancer in addition to BRCA.
Unfortunately for Veridex and for patients, the system has not been shown to affect outcomes in metastatic breast cancer, where it has been most thoroughly studied, leading ASCO to make the following practice recommendation:
The measurement of circulating tumor cells [CTCs] should not be used to make the diagnosis of breast cancer or to influence any treatment decisions in patients with breast cancer. Similarly, the use of the recently US Food and Drug Administration–cleared test for CTCs (CellSearch Assay; Veridex, Warren, NJ) in patients with metastatic breast cancer cannot be recommended until additional validation confirms the clinical value of this test.
The CellSearch system can be regarded as a first-generation approach to the use of CTCs theranostically. A second-generation approach would make use of the genotype (proteotype, metaboltype, etc) and/or phenotype of the isolated CTCs to gain insights a simple cell count cannot supply.
The discoveries in 2004 and 2005 that non-small cell lung cancer cells acquire specific mutations in EGFR that confer susceptibility or resistance to EGFR inhibitor therapy (see NEJM link for refs), allowed the possibility of using DNA isolated from CTCs to predict and monitor response to EGFR inhibitors.
Which brings us to the present day and the NEJM paper by Maheswaran et al., all colleagues at the Mass General Hospital. Their research involves use of a CTC separation and analysis system, described in at least 11 published US patent applications, the first of which was filed in December 2005. The technology has been licensed to CellPoint Diagnostics (website is essentially empty).
Here’s a schematic of the basic procedure used in the study, which involves a cell-enrichment of whole blood, followed by CTC capture using magnetic bead–conjugated antibodies against epithelial-cell adhesion molecule (EpCAM) and analysis of the EGFR gene in captured CTCs. A clump of captured CTCs against a post in the microfluidic chamber is shown in the photo following the schematic.
[Above image from US patent application 20070099207.]
[Above image from Maheswaran et al NEJM at www.nejm.org July 2, 2008 (10.1056/NEJMoa0800668)]
CTCs were identified in all patients studied. The number of CTCs isolated per mL of blood did not correlate with radiographic tumore size, suggesting factors, such as invasiveness/vascularity, that contribute more than tumor size per se to CTC number. The authors first showed that EGFR-activating and therapy-resistant mutations could be detected reliably and with high sensitivity at a single time point.
In four patients with EGFR activating mutations, detailed serial samples were available for analysis during treatment with gefitinib, an EGFR inhibitor. In all four patients, CTCs declined during genfitinib treatment and clinical progression was associated with a resurgence in CTC number, suggesting a role for CTC number in monitoring response to treatment for NSCLC.
In all patients, genotypes of circulating tumor cells evolved during treatment, with a primary EGFR-activating mutation (i.e. primary mutation) frequently detected (19/23 patients) and with emergence of the T790M drug-resistance mutation found in roughly half the patients. T790M was present at a very low allele frequency in initial tumor specimens, and its allelic frequency in CTCs increased over time, consistent with the acquisition of clinical drug resistance. Additional activating mutations were also discovered in CTCs from some patients during therapy, suggesting the emergence of different tumor clones during therapy.
In short, this emerging technology provides a sensitive method for real-time genotyping of tumors before and during treatment. The implications for clinical practice are profound, for there now is evidence that patients can be selected for tumor-specific therapies (e.g. EGFR inhibitors), administered either singly or in combination, based on analysis of a tumor specimen obtained closer to the time of treatment than has been possible from tumor biopsy; the ideal being a specimen analyzed at the point-of-decision.
Just as importantly, this study offers hope for minimally invasive serial analysis of tumor genotype during treatment, potentially offering clinicians an option for fine-tailoring specific treatments before clinical disease progression, that is before growing tumors have the opportunity to invade organs with life-altering effects.
I believe that it is now time to declare that with studies like the one highlighted here we are witnessing the emergence of genuinely informed, practical, personalized medicine for cancer. No hype needed.