Selpercatinib and Pralsetinib Induced Chylous Ascites in RET-Rearranged Lung Adenocarcinoma: A Case Series

Introduction:

Lung cancer is the leading cause of cancer-related death in both men and women with over 130,000 deaths and more than 220,000 new cases expected each year in the United States¹. Eighty to ninety percent of newly diagnosed cases are Non-Small Cell Lung Cancer (NSCLC) with 50% of those being histologically adenocarcinoma². Rearranged during Transfection (RET), a proto-oncogene coding for a tyrosine kinase receptor, was discovered in NSCLC in 2012 as on oncogenic driver^{3, 4}. RET fusion /rearrangement occurs 2 % of lung adenocarcinomas with most patients being never smokers and a median age of 61 years^{5, 6}. Tyrosine kinase inhibitors (TKI) with anti-RET activity have been effective for RET fusion-positive NSCLC treatment, and in 2020, the FDA approved two potent RET inhibitors, i.e., selpercatinib and pralsetinib^{7, 8}. Since RET-rearranged NSCLC therapy became a standard approach, there have been recent reports indicating that chylous ascites (CA) can be a rare adverse effect of RET tyrosine kinase inhibitor (TKI) treatment. Specifically, in patients receiving selpercatinib, CA was observed in approximately 7% of cases, while no occurrences were reported with pralsetinib^{9, 10}. Interestingly, it has been reported in medullary thyroid cancer patients on selpercatinib as well and no cases have been reported with pralsetinib¹⁰. In this report, we present the cases of two patients who experienced chylous ascites (CA) while undergoing treatment with RET tyrosine kinase inhibitors (TKIs). One patient was receiving selpercatinib, and the other represents the first reported case of CA associated with pralsetinib. Additionally, we delve into the discussion regarding the management of chylous ascites in the context of RET TKI therapy.

Case Presentation:

Patient One, a 65-year-old male never smoker, presented with a persistent dry cough for approximately 3 months, a small lump growth on the right neck, and 10-pound weight loss due to poor appetite. He underwent a neck computed tomography (CT) scan which showed enlarged right level 2 cervical lymph nodes in the right lateral neck, with nodes in this region measuring up to approximately 11 mm in short axis. There were enlarged mediastinal lymph nodes with a prevascular node measuring 15 mm in short axis and a right precarinal node measuring 13 mm in short axis. There was also a nonspecific 8 mm lucency within the left side of the C3 vertebral body. A chest CT showed an apparent posterior right hilar mass lesion with adjacent airspace consolidation and mediastinal lymphadenopathy. He underwent bronchoscopy with endobronchial ultrasound (EBUS) and fine-needle aspiration (FNA) sampling of four lymph nodes (levels 4L, 4R, 7, and 11R) which demonstrated metastatic poorly differentiated lung adenocarcinoma, positive for TTF1 and Napsin A and negative for CK5/6 and P63. Continued workup with a brain magnetic resonance imaging (MRI) displayed no intracranial metastatic disease. However, a whole-body positron emission tomography-computed tomography (PET-CT) showed multi-station cervical, mediastinal, and hilar fluorodeoxyglucose (FDG) - avid lymphadenopathy in addition to the FDG – avid lesion in the C7 left transverse process suspicious for osseous metastatic disease (Figure 1, 1A). He underwent an FNA to the right supraclavicular lymph node demonstrating metastatic lung adenocarcinoma and the block was sent for molecular sequencing. Lastly, a cervical and thoracic spine MRI displayed suspicious osseous metastases to C3, C7, and T1. His final stage was Stage IVB (TXN3M1c) lung adenocarcinoma. He received one dose of carboplatin plus pemetrexed before proceeding with palliative radiation therapy where 30 gray (Gy) was delivered to his spine and right neck lesions. Molecular sequencing by Foundation One resulted in a KIF5B-RET Fusion and RET rearrangement of intron 10 (Table 1). Initially, the patient commenced treatment with selpercatinib, taking 80 mg orally twice daily. In an attempt to enhance the therapy’s effectiveness, the dosage was adjusted to 120 mg in the morning and 80 mg at night. Unfortunately, this modification resulted in the onset of persistent abdominal cramping and discomfort. In an effort to alleviate these symptoms, the patient reverted to the original dosage of 80 mg twice daily, but regrettably, the abdominal discomfort persisted. Subsequently, the patient reverted to the modified dose of 120 mg in the morning and 80 mg at night in an attempt to find a suitable balance between efficacy and tolerability.

Figure 1.

Treatment response on RET-TKI for Patient One (selpercatinib) and Patient Two (pralsetinib). 1A. Patient One PET-CT at diagnosis. 1B. Patient One PET-CT after treatment. 2A. Patient Two CT Chest at diagnosis. 2B. Patient Two CT Chest after treatment.

Table 1.

Molecular profile of the two patients in this case series.

	Molecular Characteristics
	Patient One	Patient Two
Tissue NGS
Test One Results	KIF5B-RET Fusion RET rearrangement intron 10 CDKN2A loss CDKN2B loss KDM6A loss MTAP loss TMB: 4 Muts/Mb MSI: Stable PD-L1 (22C3): TPS 10% ATR S1583C (VUS) DIS3 R716S (VUS) FANCG K175N (VUS) KIT G872* (VUS) LTK E504K (VUS)	CCDC6-RET Fusion TMB: 0 Muts/Mb MSI: Stable ARID1A G1770V (VUS) BRCA2 N2113S (VUS) EMSY rearrangement (VUS) EPHB1 S13P (VUS) IDH1 V294M (VUS) MAP3K13 V60M (VUS) SYK G5S (VUS)
Test Two Results	N/A	CCDC6-RET Fusion TMB: Low MSI: Stable PD-L1 (22C3): TPS 0% ARID1A G1770V (VUS) BRCA2 N2113S (VUS)
Liquid NGS
Test Results	CHEK2 I160T TMB: Not Evaluable MSI-High: Not Detected GNA11 V204V (SYN)	No tumor-related somatic alterations TMB: Not Evaluable MSI-High: Not Detected

Abbreviations: MSI: microsatellite instability, Muts/Mb: mutations per megabase, NGS: Next-generational sequencing, PD-L1: programed death-ligand 1, SYN: synonymous mutation, TMB: tumor mutational burden, TPS: tumor proportion score, VUS: variants for unknown significance

His cancer responded well to selpercatinib therapy; however, nine months later the patient presented to the local emergency room for abdominal distention and pain. He underwent an abdominal ultrasound and showed moderate ascites. A paracentesis was performed and approximately 2200 mL of chylous fluid was removed from the right lower quadrant (Figure 2,A). The cytology on the chylous fluid was sent to the lab for testing and came back negative but no other labs were ordered on the cytology. His symptoms returned two weeks later, where a CT of the abdomen and pelvis showed enteritis, small ascites, and abdominal wall edema. He was admitted due to abdominal wall cellulitis from the prior paracentesis site and given intravenous vancomycin and ceftriaxone. He rapidly improved clinically and was discharged on oral doxycycline and cephalexin. He was instructed to hold selpercatinib and a liquid biopsy was performed demonstrating no RET fusion in the blood (Table 1). Upon returning to the clinic a month later, his abdominal distention and chylous ascites resolved so he transitioned his therapy to pralsetinib 400 mg PO OD. Three weeks after starting pralsetinib, the abdominal distention and chylous ascites returned with 2000 mL of fluid removed (Figure 2,B). Cytology on the fluid was negative for malignancy but his triglyceride levels were high at 3,800 mg/dL. He reported worse adverse side effects to the pralsetinib than the selpercatinib so the pralsetinib was discontinued and selpercatinib was restarted at the lower dose of 80 mg BID. Additionally, monthly octreotide injections (30 mg) were added to his treatment plan. He was also referred to a dietician to discuss recommendations for a high-protein low-fat diet to help manage the chylous ascites. He continued the treatment plan above with a positive treatment response, and minimal ascites three months later (Figure 1, 1B).

Figure 2.

Collected chylous ascites fluid drawn from patients. A. Patient One: Initial chlyous ascites. B. Patient One: chlyous ascites after switching to pralsetinib. C. Patient Two: Initial chlyous ascites.

Patient Two, a 48-year-old female never smoker, initially presented with a persistent dry cough of over 6 months and weight loss of several pounds over 4 months. A decade prior, the patient underwent a PET-CT which showed a lung nodule that was initially attributed to her history of Valley fever. Two years after the initial PET-CT, a follow-up CT of the chest continued to show the stability of this nodule, and no further CT scans were recommended. Seven years later, the patient began to experience a new persisting dry cough which had worsened over time, and a chest X-ray showed innumerable diffuse bilateral pulmonary nodules. Further workup with a CT chest showed innumerable pulmonary nodules throughout the bilateral middle and lower lobes (Figure 1, 2A). The initial lung nodule from early PET-CT had enlarged, with 30+ new nodules. Left lower lobe biopsy demonstrated adenocarcinoma with mucinous features and was moderately differentiated with an acinar predominant pattern. The sample was shown to be positive for TTF-1 and CK7 and negative for P40 and CDX-2. PET-CT showed numerous FDG-avid bilateral pulmonary nodules, bilateral lower cervical, left supraclavicular, bilateral mediastinal, and left hilar lymphadenopathy, with metabolic activity in the left sacrum and right posterior acetabulum. MRI of the brain demonstrated 3 supratentorial intracranial enhancing lesions at the gray-white junction measuring up to 5 mm. Molecular sequencing by Foundation One revealed a CCDC6-RET fusion and this finding was confirmed by HopeSeq (Table 1). Guardant 360 molecular liquid biopsy testing of the blood did not show any actionable mutations. Invitae Germline testing of the blood revealed a pathogenic CFTR variant heterozygous c.1210–34TG[12]T5 (intronic). Her final stage was Stage IVB (T1cN3M1c) lung adenocarcinoma. She first underwent stereotactic radiosurgery (SRS) to the three brain metastases for local control receiving a total dose of 20 Gy to each lesion. She then initiated systemic therapy on pralsetinib 400 mg PO daily with good tolerance despite increased fatigue. Her first PET-CT scan three months later displayed near complete resolution of the bilateral lung nodules and lymphadenopathy consistent with treatment response.

Over 14 months after initiating pralsetinib, the patient began to develop symptoms of bloating, distension, and pain in the abdomen. A paracentesis was to be performed but no fluid was seen so the procedure was cancelled. Three months later, a CT of the chest, abdomen, and pelvis showed a moderate volume of ascites present. A paracentesis was immediately arranged and 850 mL of milky white ascitic fluid was removed (Figure 2,C). The fluid cytology was negative for malignancy, but her triglyceride levels were high at 1,664 mg/dL. Her pralsetinib dose was reduced to 300 mg PO daily for better tolerance. She continued to be on this dose with a good response and some mild intra-abdominal residual ascites four months later (Figure 1, 2B).

Discussion:

The RET gene is located on Chromosome 10q11.2 and encodes for a transmembrane glycoprotein receptor tyrosine kinase¹¹. Activation of the receptor requires binding to extracellular protein ligands such as glial-derived neurotrophic factors (GNDF) resulting in dimerization of the RET receptor and kinase-mediated phosphorylation of various locations in the tyrosine domain, most importantly of which is the Y1062 docking site¹². This activation sequence results in the amplification of the MAPK, JAK-STAT, and PI3K-AKT-mTOR intracellular signaling and transcription pathways that ultimately promote cell proliferation^{2, 11}. Normal activation and function of the RET tyrosine kinase receptor is crucial to the full development of enteric and renal/urinary tract tissue during human embryonic development¹³. Aberrant activation may result from genetic point mutations or gene fusions². Since its discovery, the RET gene and receptor have been implicated in multiple cancers including, NSCLC, papillary thyroid cancer, spitzoid tumors, chronic myelomonocytic leukemia, colorectal, breast, ovarian and head and neck cancers^{4, 11}.

Nearly 99% of all RET fusions are between two fusion partners with the majority being the KIF5B fusion partner (83.6%) and the other being CCDC6 (15.1%)¹⁴. RET fusion partners increase RET kinase activity by 2 to 30-fold in lung tissue resulting in unchecked cell motility, proliferation, and survival¹⁴. Next-Generation Sequencing (NGS) can detect both RET fusions and additional mutations, while RT-PCR and FISH can detect RET fusions they each have their own limitations with RT-PCR unable to detect any unknown new fusion partners and FISH unable to identify specific fusion partners^{15, 16}. Patients with RET fusion positive NSCLC tend to demonstrate adenocarcinoma on tissue biopsy and are relatively young (less than 60 years of age) and have a history of never or light smoking⁶. Around a quarter (25%) of RET fusion patients have central nervous system (CNS) involvement at presentation².

Early on, multikinase inhibitors (MKIs) with ancillary RET activity such as sorafenib, cabozantinib, and vandetanib were used to target RET fusions, however, these first-generational TKIs were severely limited due to the lack of specificity resulting in substantial “off-target” toxicities and low objective response rate (ORR) (16–53%)^{5, 17}. RET-specific TKIs were developed to decrease toxicities, increase efficacy, and penetrate the CNS, and in 2020 the second-generation RET-specific TKIs selpercatinib and pralsetinib were approved by the FDA for RET fusion NSCLC based on the LIBRETTO-001 and ARROW clinical trials^{7, 8}. ORR in patients previously treated with a platinum-based chemotherapy regimen was 61% for pralsetinib and 64% for selpercatinib and the ORR in the treatment-naïve cohort was 70% and 85% respectively. Both agents demonstrated high CNS activity, a significant improvement over MKIs, with an intracranial ORR between 85–91% for selpercatinib and 56% for pralsetinib but all nine patients had shrinkage in their CNS lesions. The most common high-grade treatment-related adverse events (TRAE) for selpercatinib were hypertension (14%), elevated alanine transaminase (ALT) (13%), and elevated aspartate aminotransferase (AST) (10%), while neutropenia (18%), hypertension (11%), and anemia (10%) were the most common for pralsetinib. The majority of TRAEs with selpercatinib or pralsetinib did not require a drug interruption; however, 30% (selpercatinib) and 38% (pralsetinib) needed a dose reduction, while 2% and 6% had to discontinue treatment. In contrast to other TKIs, both selpercatinib and pralsetinib exhibit a unique binding conformation with the RET receptor resulting in high affinity binding and afford more efficient blocking of phosphorylation; this binding mode avoids the development of gatekeeper resistance mutations but is susceptible to non-gatekeeper-resistance mutations in a cross-like manner between these TKIs¹⁸.

Chylous ascites (CA) is a distinct form of ascites characterized and diagnosed by a cream or milk-like fluid consistency with a triglyceride level over 200 mg/dL¹⁹. CA forms in the peritoneal cavity due to trauma and rupture of the lymphatic vessels or increased peritoneal lymphatic pressure from an obstruction resulting in the leakage of lipid-rich lymph²⁰. CA is very rare with an incidence of about 1 per 20,000 admissions over the course of 20 years, yet incidence rates have increased due to extended survival in cancer patients²¹. CA-associated causes are categorized as traumatic, congenital, infectious, neoplastic, postoperative, cirrhotic, and cardiogenic with almost two-thirds of all CA cases in developed countries being associated with abdominal malignancy and cirrhosis²². Management involves diagnosing the underlying cause which may require paracentesis, laparoscopy and peritoneal biopsy, lymphangiography, lymphoscintigraphy, and CT/MRI imaging²². Reducing chylomicron flow through the thoracic duct may be achieved by a prescribed high-protein diet combined with restriction of long-chain triglycerides (low-fat)²³. Pharmacological interventions may be required if CA cannot be managed through diet alone. Somatostatin analogs like octreotide and lanreotide or other agents such as orlistat and etilefrine can be utilized by increasing vascular resistance thus reducing lymphatic system flow or inhibiting fat reabsorption in the intestines²³. Bowel rest and total parenteral nutrition may be necessary for patients who remain symptomatic and do not respond to dietary or medical therapy; this reduces the lymph flow by bypassing the bowel^{22, 24}. Non-conservative measures such as repeated paracentesis, lymphangiography with embolization, peritoneovenous shunting, or other surgical interventions should be applied as a last resort²³.

Recently, a multicenter retrospective study by Kalchiem-Dekel et al. evaluated the frequency, clinicopathological features, and management of chylous effusions (pleural or peritoneal or both) in 7517 pan-cancer patients treated with MKIs or RET-TKI⁹. Overall, 22 of these patients developed a chylous effusion with the median age was 66 years, and they were predominantly male (68%), never smokers (73%), and most patients had lung adenocarcinoma (55%) or medullary thyroid cancer (23%). Chylous effusions were most common with selpercatinib (7%) followed by agerafenib (4%), cabozantinib (0.3%), and lenvatinib (0.02%); none were observed with pralsetinib. The time from RET-TKI initiation to the first radiographic evidence of effusion for selpercatinib was 1.6–19.5 months, however, the vast majority of patients (86%) were diagnosed with having a chylous effusion one year or more into therapy. Only 13% of patients were positive for malignant cells present after the cytologic analyses of the chylous fluid. This retrospective analysis was crucial in highlighting the importance of continued observation for rare toxicities from TKIs. A review of literatures shows sporadic cases of chylous effusions with TKI’s like dasatinib²⁵. Chylothorax is an exceptionally uncommon occurrence with TKI’s, and its underlying mechanisms remain inadequately elucidated. Several hypotheses have been proposed, primarily centered around the pharmacodynamics of the drug. One plausible explanation involves the microscopic disruption of lymphatic channels, rather than direct obstruction of the thoracic duct, as the etiological factor behind chylous effusion in our patient. Gastric bowel wall edema has also been reported with selpercatinib²⁶.

We observed chylous ascites in our two patients’ cases months following the publication of their study. In our case series, both of our patients were never smokers and had lung adenocarcinoma histology. Each patient was identified with a RET fusion at diagnosis with Patient One harboring the KIF5B fusion partner and Patient Two the CCDC6 partner. Both patients’ triglyceride levels far exceeded the 200 mg/dL threshold for diagnosis at 3,800 mg/dL and 1,664 mg/dL. Patient One’s first evidence of CA was after 9 months of therapy with selpercatinib while Patient Two’s CA appeared after 14 months of initiating therapy with pralsetinib, the first time described. Furthermore, Patient One was discontinued on selpercatinib and switched therapies to pralsetinib with worsening CA symptoms. He later switched back to a reduced dose of selpercatinib, started a high-protein low-fat diet, and began monthly octreotide injections. Patient Two’s CA resolved after reducing the pralsetinib dose. Despite the CA, each patient continued therapy after 17 months and 26 months with no evidence of active disease. Nevertheless, the RET-TKI-induced pathological mechanism causing CA is unresolved.

Clinical Practice Points:

• The selective RET-TKIs, i.e., selpercatinib and pralsetinib are effective against RET fusions in patients with NSCLC.

• Chylous ascites is an extremely rare adverse toxicity seen in patients on RET-TKIs and may be confused with the progression of disease.

• Imaging, paracentesis, and fluid triglyceride levels are crucial in diagnosing chylous ascites.

• RET-TKI dose reduction, a high-protein low-fat diet, and monthly octreotide injections are all effective management options for patients with RET-TKI-induced chylous ascites.

• The mechanism of chylous ascites in patients treated with RET-TKIs remains poorly understood.