Comparative Evaluation of [^99mTc]Tilmanocept for Sentinel Lymph Node Mapping in Breast Cancer Patients: Results of Two Phase 3 Trials

In this pooled analysis of breast cancer patients participating in 2 highly similar phase 3 trials, [^99mTc]tilmanocept’s identification of lymph nodes was highly concordant with identification by VBD. Of the 209 lymph nodes identified by VBD, 207 were also identified by [^99mTc]tilmanocept (p < 0.0001). Interestingly, VBD failed to identify a SLN in 17 of 148 breast cancer patients (11.5 %). This was less than the reported failure rate for VBD.17 Within these 17 patients, [^99mTc]tilmanocept identified at least 1 node in all but 2; notably, these patients also had no blue nodes. There were significantly (p < 0.0001) more radioactive nodes (SLN = 320) than blue nodes (SLN = 209). Of 320 radioactive nodes, only 207 (64.69 %) were also blue. Similarly, in only 78 of 146 patients (53.42 %) with at least 1 radioactive node were all the radioactive nodes also blue. Our findings are consistent with those of Krag and coworkers, where the isotope was better than VBD in all aspects.18

[^99mTc]Tilmanocept’s ability to identify more cancer-containing nodes than VBD (31 vs. 25, p < 0.0312) demonstrates that [^99mTc]tilmanocept provides a benefit in intraoperative lymphatic mapping beyond that provided by VBD alone. Recent reports that local recurrence and survival are not affected by full node dissection after positive sentinel node biopsy in breast conservation patients accentuates the importance of the performance of the [^99mTc]tilmanocept SLN mapping agent in these studies and points to the need to accurately assess lymph nodes in order to optimize postsurgery management of the patients.1 ^, 19 ^, 20 Our findings suggest, then, there is significant clinical utility in knowing which patients have multiple positive sentinel nodes.

Entrance of a patient into the clinical trial was at the discretion of the surgeon. As long as the patient did not have metastatic disease, clinically positive lymph nodes, or any of the exclusion criteria listed in Table 1, the patient was eligible for the study. If the 2 T4 patients were excluded the concordance would be slightly higher and the performance of VBD would appear to be better. This is because in one of the patients VBD failed to stain a disease-positive lymph node. Two patients were stage T4. Both patients had 2 “hot” lymph nodes. Both nodes of one patient were disease-positive with 1 of the lymph nodes not stained blue. The other patient had a “hot” and “blue-stained” lymph node that was disease-positive; the second “hot” lymph node was disease-negative and not stained blue. Based on criteria other than T staging, for example, N0, M0, there is no reason to exclude such patients.

There is a growing discussion as to whether or not a complete node dissection is necessary.3 ^, 19 ^, 21 [^99mTc]Tilmanocept identified numbers of patients who had additional pathology-positive sentinel nodes, while VBD did not. A study site, University of California, San Diego, had 20 breast cancer patients with pathology-positive nodes identified with at least 1 radioactive and/or blue sentinel node; 16 had only a single radioactive and blue positive sentinel node. Also, four patients had additional radioactive pathology-positive nodes that were not blue, deeming them “multinode positive patients.” The indication that [^99mTc]tilmanocept may be able to better identify multinode pathology-positive patients may be important to the future of breast cancer treatment. Specifically, extended field radiation therapy will be offered to these patients if not full node dissection. To the extent that [^99mTc]tilmanocept accumulates at that the target tissue based on a specific biochemical interaction (CD206 receptor-targeted), this molecule represents the current generation of imaging technology and is specifically designed to match the biochemical requirements of the clinical problem. Thus, the increase in found disease represents a notable finding related to the SLN procedure, especially where this is translated to a large patient population. If blue dye alone were used, the addition of tilmanocept could potentially change the diagnosed number of positive nodes in 1.35 % of our nation’s 190,000 or so breast cancer patients, which is roughly 2,000 patients per year. Since adjuvant decisions are now often based on sentinel node biopsy results alone, accurately knowing those patients who have multiple positive nodes may change radiation and systemic therapy choices.

Vital blue dye was chosen as the comparator in these studies because it is routinely used in current practice to identify SLNs during breast cancer surgery and has a history as a validation agent for SLN biopsy.6 ^, 22 Radioactive colloids have been used off-label with the intent to augment VBD.7 ^, 8 ^, 23 However, [^99mTc] sulfur colloid injection (SCI) has recently been granted FDA approval in the United States for lymphatic mapping during breast cancer operations only. Nevertheless, radiocolloids remain an unstandardized SLN mapping agent, mitigating their utility as a valid comparator agent. VBD usage permitted a rigorous, within-patient experimental design, significantly increasing the statistical power. There is a possibility despite finding one blue node and finding several hot nodes, that a rare additional aberrant blue node was missed somewhere. It is also possible when no blue node was identified at all and several hot nodes were found, that again a distant blue node may have been overlooked. However, most of the positive axilla did go on to full node dissection, and no further blue nodes were identified.

These phase 3 studies provided a basis with which to compare the performance of the radiolabeled colloids via meta-analysis. A recent review of SCI was based on a meta-analysis of 15 studies involving 9,213 breast cancer patients receiving both SCI and VBD.24 In the meta-analysis, SCI and VBD detected at least 1 lymph node in 94.1 and 85.1 % of patients, respectively. In the current studies, [^99mTc]tilmanocept and VBD detected at least 1 lymph node in 98.6 % (146 of 148) and 88.5 % (131 of 148) of patients, respectively. Lastly, we constructed a meta-analysis of these phase 3 trials and contrasted the results against the recently published selected performance data of Nanocoll.25 Tilmanocept had superior SLN localization rate (99.9 vs. 95.1 %) and a higher degree of localization (2.16 vs. 1.66 SLN per study) (p < 0.0001). Such comparisons will be duplicated with larger sample sizes if [^99mTc]tilmanocept becomes commercially available.

These phase 3 trial and the previous phase 1 and 3 studies provided preliminary data regarding the performance of [^99mTc]tilmanocept compared with the current SLN agents. First, [^99mTc]tilmanocept is a wholly standardized synthetic molecule with a defined molecular structure that permits rapid uptake into lymph nodes. When imaged within 10 min postinjection, at least 1 “hot” lymph node was detected in 98 % of the early images. Second, based on physician use reports, the administration of [^99mTc]tilmanocept is associated with significantly less pain and discomfort than for radiolabeled colloids. Third, based on the contrasts of dosimetry and preoperative imaging, [^99mTc]tilmanocept exits its injection site significantly quicker than radiolabeled colloids, creating less shine-through, thus facilitating better step-down for observation of SLNs that reside near injection sites.15 Finally, both in vivo imaging and in vitro analysis of [^99mTc]tilmanocept’s receptor binding properties indicate it binds specifically to SLNs and does not move downstream to distal lymph nodes, permitting high confidence that a “hot” lymph node found during a “next-day” surgery is a sentinel lymph node.9 ^, 15 ^, 26 This factor also allows the patient to be imaged as early as 10 min and as late as 24 h or more after injection. These attributes will translate into greater scheduling flexibility in nuclear medicine and operating room without sacrificing either short-term or long-term SLN detection success.

[^99mTc]Tilmanocept is a modern radiopharmaceutical designed on a chemical platform that permits future development.26 Technetium-99m-labeled antimony sulfide colloid and sulfur colloid, both of which were developed more than 35 years ago, cannot be chemically modified.27 ^, 28 The chemical design of tilmanocept permits the facile attachment of additional imaging reporters, such as fluorescent dyes.29 ^, 30 Routine optical imaging via robotic-assistance or intraoperative hand-held imaging will require the covalent attachment of the fluorescent tag to the molecular imaging agent.31 ^, 32 Optical imaging would facilitate intraoperative administration, which may be more compatible with SLN mapping of lung, GI, and GU cancers. The DTPA chelators of tilmanocept (Fig. 1) permit radiolabeling with different radioisotopes, such as gallium-68 or indium-111 or paramagnetic atoms.33 Gallium-68, which is a generator-produced positron-emitter, enables PET/CT hybrid imaging.34 PET imaging provides higher sensitivity and spatial resolution, as well as greater scatter and attenuation correction than SPECT. Hybrid imaging with PET/CT may prove to be more compatible for preoperative pelvic imaging of prostate cancer. Lastly, Nanocoll is a microcolloid of human serum albumin and must be tested for antibodies to HIV and hepatitis C, as well as hepatitis B surface antigen.35 Rigorous testing and increased regulatory compliance add expense to the manufacturing of Nanocoll and substantially decreases the likelihood that modifications for performance improvements will be commercialized. Tilmanocept is not derived from blood and therefore can be synthesized at very high scales without continual monitoring for new biological “threats” from the human blood supply.

There were two reasons for using the “3-sigma” rule. First, the “3-sigma” rule is based on a statistical rationale and is used extensively in the radiation sciences to define a criteria known as the “Minimum Detectable Activity.”36 When a measurement is greater than background by 3 times the standard deviation of the background, the probability that the measurement is truly different than background is greater than 99.7 %. The measurement of radioactivity is a counting process that follows a Poisson distribution, which occurs when the process is the result of many attempts and very few successes. The standard deviation of a Poisson distribution is the square root of the measurement. Consequently, the Minimum Detectable Activity for a sentinel lymph node is 3 times the square root of the background count rate, which can be obtained by a single measurement. Second, the “10 %” was empirically established for radiocolloids and may not be applicable to a nonparticulate receptor-targeted radiopharmaceutical.18 Due to its small size (7 nm) and high receptor avidity, tilmanocept delivers significantly more radioactivity to the sentinel lymph node and clears from the injection site 5-fold faster than sulfur colloid. For example, the UCSD breast cancer patients were studied with a protocol that used a single intradermal injection of 0.1 ml. A total of 95 patients had at least 1 “hot” lymph node. The count rate of the “hottest” lymph node from each patient ranged from 375 cps to an excess of 999,999 cps, which is the readout maximum of the Neo200 counting system. Seven lymph nodes exceeded this count rate. The median count rate of the “hottest” lymph node was 15,738 cps. Appling the “10 % rule” when the “hottest” node has 15,000 cps would permit the surgeon to stop if the surgical bed had a count rate of 1,500 cps or less. This would risk leaving behind a lymph node that could be 1,500 cps and by radiocolloid standards would be considered very “hot.” Such a lymph node would be almost 3 orders-of-magnitude higher than background and would more than satisfy the “3-sigma rule,” which was our statistical basis for defining a radioactive signal and, hence, a sentinel lymph node.

A sentinel lymph node definition based on the technical criteria of “Minimal Detectable Activity” may not provide the most efficient “stopping rule” for the mapping procedure. Although a “stopping rule” for a receptor-targeted agent such as [^99mTc]tilmanocept will require a separate study and may not be at all similar to the rule used for particulate radiotracers, a comparison of our results to the “10 % rule,” which is used for unfiltered and filtered [^99mTc]labeled sulfur colloid may be instructive.18 ^, 37 ^– 40 The “10 % rule” and the “3-sigma rule” identified the same number of patients (n = 146) with at least 1 “hot” lymph node. Consequently, the per-patient concordance rate was the same using both rules. Additionally, defining the sentinel node by the “10 % rule” did not alter the pathology status on a per-patient basis. Also, 17 fewer sentinel lymph nodes were defined as “hot” based on the “10 % rule.” Of these, 11 nodes were blue, and 2 of the 11 were pathology-positive.

In summary, these studies demonstrated that SLN detection by [^99mTc]tilmanocept is highly concordant to SLN detection by VBD. [^99mTc]Tilmanocept detected more SLNs than VBD, and these additional SLNs enhanced the detection of clinically important nodal metastases. Given these results, this agent is safe and provides a modern radiopharmaceutical for future advances in sentinel lymph node mapping.