Pafolacianine for imaging of pulmonary neoplasms: a promising supplement for modern surgical technique

Dr. Sarkaria and colleagues present a phase 3 clinical trial examining the utility of pafolacianine for the intraoperative evaluation of lung cancers (1). Pafolacianine is a first-in-class conjugated folate-receptor (FR) analog that behaves similarly to indocyanine green (ICG) (1). Following administration, the chemical binds folate receptors with a 1 nM affinity and is removed from folate receptor-negative tissue in less than 30 minutes (1). The lung is then exposed to near infrared (NIR) light that excites the pafolacianine molecules in situ. This results in fluorescence that becomes visible with an intraoperative camera system (1).

Participants with suspected or biopsy-proven cancer were randomized in a 10:1 proportion to undergo intraoperative molecular imaging (IMI) or a standard “white light” surgical approach; a ratio requested by the Food and Drug Administration (FDA). There were a total of 112 patients, 100 experimental patients, 11 “white light” patients, and 1 patient who withdrew consent due to an infusion reaction. Each participant was administered 0.025 mg/kg IV of pafolacianine within 1–24 hours before surgery. The lung was initially inspected with thoracoscopy and palpation, and then re-inspected in the experimental group for fluorescence. Both the primary nodule and the remainder of the lung were evaluated for other possible synchronous lesions. Further resection was performed on observed fluorescence at the discretion of the surgeon.

The primary endpoint was a clinically significant event (CSE), referring to a finding that meaningfully changed the intended surgical operation. This was defined as (I) removal of one or more primary “lung nodule(s)” detected only by NIR (and not by visual inspection or palpation); (II) removal of one or more synchronous cancer lesions detected only by NIR; and (III) identification of a positive margin <10 mm under NIR.

Results

The researchers found that 1 or more CSEs occurred in 53 of 100 subjects (53%) compared to a prespecified limit of 10% (P<0.0001). Of these, 43 had only 1 CSE and 10 others had more than 1 for a total of 65 CSE among experimental subjects (1).

In 19 subjects, IMI located a primary nodule that could not be found by visualization or palpation. Another 11 synchronous occult lesions were found in 9 subjects. Of these patients, 4 had lesions in a different lobe from the primary nodule and may have been missed with a traditional approach. Of the synchronous lesions discovered only with IMI, 8 of the 11 were outside the planned resection field. All were removed with a wedge resection (1).

Surgical margins ≤10 mm were found in the remaining 38 CSEs on back table imaging. These findings were confirmed in 32 of the 38 specimens (84.2%) (1). Of the 40 patients with margins measured ≥10 mm, 15 specimens (37.5%) were measured by a pathologist to be ≤10 mm. A change in the overall scope of the surgical procedure occurred for 29 subjects (22 increase, 7 decrease).

The sensitivity for detecting cancerous tissue in this study was 80 out of 104 (76.9%). The false positive rate was 28 of 108 (25.9%). These included 10 primary nodules and 18 synchronous lesions. The final histology on the false positive tissue specimens was normal lung parenchyma or benign tissue (1). These included granulomatous inflammation, fibrous tumor, a “meningothelial-like” nodule, one anthracotic nodule, and one lipoid pneumonia (1).

Overall, in patients with suspected or confirmed cancer with known positive FR expression, sensitivity was 77.2% and the false positive rate was 21.1%. The false positive rate among participants with confirmed cancer alone was 17.5%. This reflects a rate of 1.4% for primary nodules, and 59.7% for synchronous lesions. Finally, among the 113 specimens with folate receptor-positive specimens, 91 displayed fluorescence (80.5%).

The authors were transparent about several inconsistencies in dosing—including partial dosing, missing dosing information, or interrupted dosing—that may have affected outcomes. No major drug-related adverse effects occurred. Minor reactions occurred in 19 patients: including nausea (10 patients), hypertension (4 patients), vomiting (3 patients), abdominal pain (2 patients), flushing (2 patients), transient hypotension (2 patients), pruritis (1 patient), and urticaria (1 patient). Pafolacianine infusion was interrupted in 11 patients and stopped in another 5 patients. The authors also note that 524 adverse events were recorded for 110 of the 112 patients, but claim that the majority were related to the surgery and not pafolacianine (1).

Discussion

This is a well-organized study that describes promising results from a first-in-class medication. The researchers should be commended that CSEs occurred in more than half of experimental patients. This is strong evidence that pafolacianine should be studied further, and that IMI may have widespread utility in sublobar resection.

Of particular note, surgeons were able to identify a primary nodule with IMI that could not otherwise be found in nearly 20% of patients. This is especially noteworthy for the future of surgery being performed on early-stage cancers, and non-solid ground glass lesions.

Perhaps the most sobering finding was that 11 additional synchronous lesions were found only with IMI in 9 patients (9%). Worse, nearly half of these lesions (n=4) were in different lobes, and 8 (73%) were outside the planned resection field. In their introduction, the authors correctly emphasize the potential for this technology to reduce recurrence and increase the efficacy of sublobar resection (1).

Unfortunately, there were also several issues with this study. The authors describe that 85% of lung cancers express folate, but that means 15% do not. Further, according to this study, only 80% of FR positive tissue will display fluorescence; or roughly only 68 of these 85 of patients (80%) that even express folate receptors. As a result, 32% of all patients with lung cancer will not have reliable IMI and will be at high risk for false negative results. In this study, resection of the fluorescent lung was based on surgeon judgement, which will be highly variable with widespread use. Objective guidelines should be established to define thresholds that are concerning for malignancy; but this may not be an appropriate expectation for a phase 3 trial.

Since the purpose of pafolacianine is to supplement—and not replace—surgical judgement, it is unclear how this will impact patient outcomes. Sublobar resections that utilize IMI could still be performed with established techniques only with additional steps. On the other hand, it may dissuade surgeons from performing resection if lesions are not biopsy-proven. It may also compromise cases that depend on supplemental assistance. Robotic sublobar resection would be at particularly high risk since palpation is lost and other adjuncts (e.g., wire placement, fiducial, etc.) would have had to be performed before surgery.

I therefore do not share the authors’ confidence that folate receptor testing is “not required” (1). Given the risks of false negativity, pre-operative knowledge of folate-receptor status would be critical for reliable use. It is also unclear how there could be any false positives in the 1.4% of primary nodules in patients with confirmed cancer.

Prediction of surgical margins was also disappointing. Of the 40 patients with margins measured >10 mm, nearly 40% were found to be <10 mm in pathologic analysis. Of the 38 patients with margins <10 mm, 15.8% had margins >10 mm. This means 21 patients of 78 patients noted to have margin concerns displayed features incorrectly predicted by IMI. An operative-to-pathologic concordance of 27% is significantly worse than data published in the Initiative for Early Lung Cancer Research on Treatment (IELCART) (2). Wolf and colleagues show that measuring tumor with only a needle and ruler, the Pearson correlation coefficient between surgeon measurement and pathologic analysis was 0.94 overall, with 0.99 for part-solid, and 0.95 for solid tumors (2).

Many of the 524 adverse events published by the authors may be related to the surgery, but without further study, it is unclear to what extent these were exacerbated by a first-in-class medication.

The authors mention that pafolacianine is cleared from the tissue “sufficiently rapidly”, and has a reliable tumor-to-background contrast ratio for operative visualization (3). Unfortunately, the mechanism is never described in the paper or the referenced literature. While the authors do mention that pafolacianine is cleared from folate-receptor negative tissue within 30 minutes, they do not clarify the pharmacokinetics in the multitude of folate-receptor positive tissue. At minimum, this includes their list of involved metastatic lesions: colorectal, breast, renal, melanoma, spine, ovarian, prostate, pancreatic, liposarcoma, and neuroendocrine.

The study also describes that folate receptor expression is unaffected by chemotherapy; presumably to reinforce its use in patients who undergo induction treatment (1). However, the reference used to support this claim examined the behavior of receptor expression in ovarian cancer (4). It is unclear if the behavior of ovarian cancer will respond similarly to lung cancer, especially considering divergent chemotherapeutic regimens. Fortunately, patients undergoing neoadjuvant treatment are less likely to be candidates for sub-lobar resection.

There was also a procedural concern in the study design; each patient is described as acting as their control (1). The operating surgeon began with a thoracoscopic evaluation with white light. Following a full operative survey, patients randomized to the experimental group were then evaluated with IMI. There was an additional true control group of 11 patients who received pafolacianine but did not undergo IMI. The authors state that the randomization plan was decided by the FDA and the investigators were not aware of the structure (1).

This approach lacks the reliability that comes with clearly defined experimental and control groups. However, it has the advantage of preserving the same surgical judgement and operative conditions and for each patient. There is no concern, for example, that nodules only found with IMI were the result of differences in skill between surgeons. Future studies should separate these two groups in a way that is more clearly defined. But for a first-in-class publication, this study design should not affect the integrity of the results.

One of the few diagnostic alternatives in the literature is ICG. ICG is a water-soluble, cyanine-based dye that has been available for over 60 years (5). Following intravenous injection, ICG is rapidly conjugated by albumin (and other plasma and lipoproteins) and eliminated via biliary secretion following intrahepatic deconjugation (5).

Similar to pafolacianine, the excitation and emission wavelengths are both in the NIR spectrum, making it ideal for tissue penetration (5). Unlike pafolacianine, ICG does not have a molecular affinity for individual receptors in lung cancer or any other tissue. ICG is commonly used to identify vascular structures during its 120–240 second half-life, and the hepatobiliary tree during delayed elimination (5).

ICG-enhanced tumor identification in lung resection has been reported since at least 2014 (6). Because it has no ligand-based affinity, its deposition in tissue is likely due to the enhancement permeability and retention (EPR) effect which was described by Matsumura and colleagues in 1986 (7) This behavior relies on defective endothelial cells with fenestrations greater than 600–800 nm present in cancerous tissue (6,7). The space allows for both the ICG and its conjugated carrier protein to escape the endothelial lumen into extravascular locations. EPR was originally conceived to describe the localization of macromolecular chemotherapeutic regimens and not specifically for ICG (7). It also exposes a theoretical weakness in the efficacy of ICG. Namely, multiple pathophysiologic mechanisms can result in exudation of large vascular content, including nearly all inflammatory processes.

Unfortunately, much of the remaining application of ICG requires the occlusion of segmental or lobar arteries before systemic administration (5,8). The absence of fluorescent enhancement assists with the identification of segmental or lobar anatomy. But if selective arterial occlusion is required, it is difficult to appreciate the benefit for structures that have already been identified.

Pafolacianine has several theoretical advantages over ICG. First, it has a ligand-based affinity for many lung cancers. Since ICG is dependent on the development of non-specific fenestrations between endothelial cells, it has limited capacity to delineate between benign and malignant conditions. Moreover, inflammation caused by surgical dissection will likely affect the reliability of ICG during the operation.

ICG has the natural advantage of 60 years of clinical experience that gives a far better sense of short- and long-term side effects. The EPR effect would also likely allow fluorescence in tissue that would be missed by pafolacianine; either due to an absence of folate receptors or the 20% of non-fluorescing receptor-positive targets. However, this advantage would be counterbalanced by the expected false positives from benign inflammatory processes, autoimmune disease, infectious foci, and tissue traumatized by surgical manipulation.

A trial comparing these two modalities, or investigating how combined use might balance their limitations, might benefit the future of the field.

Conclusions

In the era of lung cancer screening, the discovery of early cancers, ground glass lesions, and partially solid nodules will continue to increase. Additionally, there is mounting evidence supporting the use of sublobar resection for early-stage peripheral lesions (9). Any techniques that can enhance intraoperative evaluation may be critical for these patients. This is especially true when targets are small, located far away from minimally invasive incisions, not palpable, or removed with robotic approaches that limit or preclude intraoperative palpation.

As with any new technique, further study is required; especially to (I) establish guidelines for tissue luminosity that requires resection; (II) improve prediction of appropriate margins; (III) further evaluate the short and long-term side effects; and (IV) confirm the types of metastatic cancers that express folate receptors. To their credit, the authors describe the virtue of this medication well. Pafolacianine offers the possibility of significantly improving outcomes of lung cancer surgery by enhancing intraoperative visualization of primary nodules, synchronous lesions, and surgical margins. When used as intended, it has the potential to be a strong supplement to current imaging and intraoperative judgement. Despite the limitations expected in any new technique, this is a highly promising technology that may very well improve patient outcomes and survival.

Acknowledgments

Funding: None.

Footnote

Provenance and Peer Review: This article was commissioned by the editorial office, AME Clinical Trials Review. The article has undergone external peer review.

Peer Review File: Available at https://actr.amegroups.com/article/view/10.21037/actr-23-13/prf

Conflicts of Interest: All authors have completed the ICMJE uniform disclosure form (available at https://actr.amegroups.com/article/view/10.21037/actr-23-13/coif). The authors have no conflicts of interest to declare.

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