Pneumocystis jirovecii pneumonia in a patient with HR⁺/HER2^‑ advanced breast cancer following abemaciclib combined with endocrine therapy: A case report

Introduction

Pneumocystis jirovecii infection, classified as Pneumocystis jirovecii pneumonia (PJP), is a leading cause of pneumonia in immunocompromised populations, including individuals with human immunodeficiency virus (HIV), solid organ transplant recipients on immunosuppressive regimens, and patients with cancer undergoing dose-dense chemotherapy (1). Notably, PJP carries a substantial mortality risk, particularly in non-HIV immunocompromised populations where 90-day mortality is significantly higher than in HIV-infected patients (adjusted OR=5.47; 95% CI:2.16–14.1 for solid tumor patients) (2). Owing to their immunocompromised status, these patients exhibit heightened susceptibility to Pneumocystis jirovecii colonization and subsequent progression to clinically significant PJP (3), with critical risk factors including prolonged high-dose corticosteroid use (≥10 mg/day prednisone equivalent, increasing mortality by 80%) and lymphopenia (<0.6×109/l). Mortality rates escalate to 43.3% for non-HIV PJP patients requiring ICU admission, and reach 61.6% at 3 months in lung cancer population (4).

Advanced HR+/HER2- breast cancer survival has significantly improved with CDK4/6 inhibitors (median OS now exceeding 5 years in 1L), particularly in visceral metastasis (5). Abemaciclib, a selective CDK4/6 inhibitor, is approved for the treatment of hormone receptor (HR)-positive/human epidermal growth factor receptor 2 (HER2)-negative breast cancer. As a CDK4/6 inhibitor, abemaciclib functions by inhibiting CDK4 and CDK6 proteins, which are critical regulators of cell division and growth. This inhibition directly suppresses cancer cell proliferation (6). Clinically, abemaciclib is utilized in combination with endocrine therapy (ET), demonstrating established efficacy in both early-stage and advanced HR+/HER2− breast cancer settings (7). It constitutes a first-line standard-of-care option for HR+/HER2− advanced disease, with key clinical trials (MONARCH-2 and MONARCH-3) reporting therapeutic benefits when combined with aromatase inhibitors or fulvestran (8,9). The most common adverse event (AE) associated with abemaciclib is diarrhea, with other frequently reported toxicities including nausea, fatigue, elevated serum creatinine and myelosuppression (neutropenia and lymphopenia) (10,11).

The present report details a case of PJP occurring in a patient with HR+/HER2− advanced breast cancer receiving abemaciclib in combination with ET. There have been no prior documented cases of PJP in patients treated with abemaciclib, to the best of our knowledge.

Case report

In February 2023, a 52-year-old treatment-naïve woman diagnosed with stage IV HR+/HER2− advanced breast cancer, confirmed via core needle biopsy of the primary breast lesion and metastatic lung deposits at Longhua Hospital affiliated to Shanghai University of Traditional Chinese Medicine (Shanghai, China), had been receiving combination therapy with abemaciclib (400 mg/day orally), letrozole (2.5 mg/day orally) and goserelin (3.6 mg administered subcutaneously once monthly) in 28-day cycles for 6 cycles. Histopathology and immunohistochemistry images of the primary breast lesion and metastatic lung deposits via core needle biopsy are presented in Fig. S1. Tissue sections were formalin-fixed and paraffin-embedded, and immunohistochemistry was performed using antibodies following antigen retrieval. HER2 expression was scored using the 2023 American Society of Clinical Oncology/College of American Pathologists guidelines (12).

Tissue specimens were initially fixed in 10% neutral buffered formalin, prepared by mixing 100 ml of 37–40% formaldehyde with 900 ml PBS, at 4°C for 24 h (cold fixation). Paraffin-embedded tissues were prepared and sectioned to a thickness of 4 µm. For antigen retrieval, heat-induced epitope retrieval was performed using citrate buffer (pH 6.0) at 95°C for 20 min, followed by a 30-min cooling period at room temperature (RT). Sections were then rehydrated through a descending alcohol series: Two washes in xylene (5 min each), two washes in 100% ethanol (2 min each), one wash in 95% ethanol (2 min), one wash in 70% ethanol (2 min) and finally a rinse in dH2O. For intracellular or membrane epitopes, permeabilization was carried out using 0.1% Triton X-100 in PBS for 10 min at RT. Non-specific binding sites were blocked by incubating sections with 5% normal goat serum (cat. no. 16210064; Thermo Fisher Scientific, Inc.) for 1 h at RT. Sections were subsequently incubated overnight at 4°C with the primary antibody, rabbit anti-CDK4 monoclonal antibody (cat. no. 12790; Cell Signaling Technology, Inc.), diluted 1:200 in antibody diluent. Following primary incubation, sections were incubated for 1 h at RT with the horseradish peroxidase (HRP)-conjugated secondary antibody, goat anti-rabbit IgG (H+L) (cat. no. ab6721; Abcam), diluted 1:500 in PBS. Detection was performed using HRP/DAB with 3,3′-diaminobenzidine (DAB; cat. no. K3468; Dako; Agilent Technologies, Inc.) as the chromogen.

For H&E staining, sections were stained with Harris hematoxylin for 5 min at 25°C, followed by a 10-min rinse under running tap water. Differentiation was then performed by briefly treating sections with 1% acid alcohol (1 ml HCl in 99 ml 70% ethanol) for 5–10 sec at RT; this step was monitored microscopically until the nuclei turned pale blue. Bluing was achieved by immersing sections in 0.2% ammonium hydroxide for 30 sec at RT. Sections were counterstained with 0.5% aqueous eosin Y solution for 2 min at RT. Immediately following eosin staining, sections were dehydrated through a graded alcohol series. All H&E and IHC-stained sections were routinely observed under a light microscope during protocol optimization and quality control. Finally, images were captured using a BX53 microscope (Olympus Corp.) equipped with a DP74 camera at either ×200 or ×400 magnification (oil immersion for ×400).

Before starting antitumor therapy, the baseline CD4+/CD8+ T-cell counts of the patient were normal, and viral/bacterial infection screens were negative. Tumor response assessed using CT demonstrated partial remission per the Response Evaluation Criteria in Solid Tumors version 1.1 criteria in June 2023 (13) (Fig. 1).

Figure 1. Tumors were assessed using chest CT and partial remission was indicated according to Response Evaluation Criteria in Solid Tumors version 1.1 after 6 months of treatment (left panel, baseline; right panel, 6 months; tumor site is marked by arrow).

In August 2023, the patient presented with fever, fatigue and a cough, requiring hospitalization. Chest CT revealed minimal lung metastases alongside bilateral interstitial pneumonia (Fig. 2). Laboratory investigations revealed elevated serum high-sensitivity C-reactive protein (CRP) and serum amyloid A (SAA), reduced circulating T-cell (CD4+ and CD8+) and B-cell (CD19+) counts, and negative serum (1,3)-β-D-glucan (BG). Nucleic acid amplification tests for SARS-CoV-2 and common respiratory viruses returned negative results. Detailed laboratory results are presented in Table I. The laboratory findings indicate that the patient developed grade 3 lymphopenia, grade 1 neutropenia and anemia during abemaciclib therapy. Bacterial pneumonia was initially suspected based on clinical presentation and elevated inflammatory markers. Anticancer therapy was discontinued, and the patient received levofloxacin (750mg/day intravenous) combined with piperacillin-tazobactam (4.5 grams every 6 h intravenously) as empirical antimicrobial therapy for 3 days.

Figure 2. Chest CT suggested minimal lung metastases and interstitial pneumonia in both lungs in August 2023.

Table I. Laboratory data.

Table I.

Laboratory data.

Variable	Value (Pre-PJP treatment)	Value (Post-PJP treatment)	Reference rangea
White-cell count, /µl	4,170	5,280	3,500-9,500
Neutrophil count, /µl	2,790	5,370	1,800-6,300
Lymphocyte count, /µl	590	1,260	1,100-3,200
CD4+, /Ul	344	512	430-980
CD8+, /Ul	199	287	380-670
CD19+, /Ul	15	97	128-388
C-reactive protein, mg/l	16.34	3.11	0.00–5.00
Serum amyloid A, mg/l	90.97	9.09	0.00–10.00
(1,3)-β-D-glucan	Negative	-	-
Lipopolysaccharide	Negative	-	-
SARS-CoV-2	Negative	-	-
Respiratory syncytial virus	Negative	-	-
Adenovirus	Negative	-	-
Influenza virus	Negative	-	-
Mycoplasma	Negative	-	-
Chlamydia	Negative	-	-
Legionella pneumophila	Negative	-	-
Human immunodeficiency virus	Negative	-	-

a Reference ranges are for healthy adults who are not pregnant.

After 3 days, the temperature of the patient increased to 39.6°C with severe hypoxia (SpO2, 71% on room air). High-flow nasal cannula oxygen therapy was initiated and bronchoalveolar lavage (BAL) was performed. Metagenomic next-generation sequencing (mNGS) of BAL fluid identified P. jirovecii and an ultra-high pathogen load (read counts of >500 with 72.87% coverage). All other pathogens, including bacteria, Mycoplasma, Chlamydia, Mycobacterium, parasites and DNA viruses, were excluded using mNGS. No malignant cells were detected in the BAL specimen. The laboratory findings indicate that the patient's lymphocyte count significantly decreased from 1.5×109/l pre-treatment to 0.5×109/l during therapy, accompanied by a decline in CD4+ T-cells as well (Fig. 3). Based on these findings, a diagnosis of PJP was established and the therapeutic regimen was switched to trimethoprim-sulfamethoxazole (TMP/SMX; 160/800 mg orally every 6 h). Concurrently, methylprednisolone was administered at 500 mg daily. The fever resolved to 36.8°C after 4 days of targeted therapy. Clinical symptoms (cough, dyspnoea and fatigue) and laboratory parameters (CRP, SAA and lymphocyte counts) demonstrated gradual improvement during the first week (Table I). The patient was discharged for outpatient management after 14 days of hospitalization. TMP/SMX was continued for 21 days, whilst methylprednisolone was tapered and discontinued over 4 weeks. The patient remained afebrile without recurrence of fatigue, chest pain, dyspnoea or cough.

Figure 3. Lymphocyte, CD4+ and CD8+ cell count curves during abemaciclib treatment. PJP, Pneumocystis jirovecii pneumonia.

Follow-up chest CT after 1 month revealed partial resolution of interstitial infiltrates (Fig. 4). Antitumor therapy was resumed 2 months later, with abemaciclib permanently discontinued in favor of fulvestrant (500 mg intramuscular subcutaneously once monthly) + goserelin (3.6 mg administered subcutaneously once monthly).

Figure 4. Chest CT showed partial resolution of interstitial inflammation in September 2023.

Discussion

The present report describes the first documented case of PJP occurring in a patient with advanced breast cancer receiving abemaciclib therapy, to the best of our knowledge. Lymphopenia is established as an adverse effect of CDK4/6 inhibitors, including abemaciclib, palbociclib and ribociclib. These agents operate through a shared mechanism of action targeting CDK4/6, which regulates cell cycle progression in both malignant cells and normal lymphocytes. CDK4/6 inhibitors serve a pivotal role in regulating the G1-to-S phase transition during the cell cycle, a process critical for T-cell clonal expansion following activation (14). Abemaciclib exerts potent inhibitory activity against CDK4/6, leading to hypophosphorylation of the retinoblastoma protein, sustained E2F transcription factor suppression and resultant G1-phase cell cycle arrest in lymphocytes (15). This mechanism directly impairs T-cell proliferation and reduces circulating lymphocyte counts. Notably, abemaciclib has been associated with markedly higher incidences of severe lymphopenia compared with palbociclib and ribociclib. This difference may relate to the greater selectivity of abemaciclib for CDK4 over CDK6, which could preferentially affect lymphocyte subsets dependent on CDK4 signaling (16). To date, there are no documented cases of PJP linked to palbociclib or ribociclib use in the published literature, to the best of our knowledge.

PJP is associated with elevated overall mortality in hospitalized populations, and the present case underscores the necessity of considering Pneumocystis as a differential diagnosis in patients receiving abemaciclib who develop pulmonary infections. PJP remains a predominant cause of pneumonia in immunocompromised hosts, particularly among HIV-infected individuals (17).

Lymphopenia is documented as a frequent AE in prior clinical trials of abemaciclib. In the MONARCH 2 and 3 trials, lymphopenia occurred in 52.7–62.9% of patients treated with abemaciclib (compared with 25.6–31.7% with the placebo). Moreover, 7.9–12.2% developed grade ≥3 lymphopenia compared with 1.8% in the control arm. Furthermore, MONARCH 2 reported a higher incidence of any-grade infections in the abemaciclib group (42.6%) compared with in the placebo group (24.7%), although infection types were not characterized but predominantly low-grade. In MONARCH 3, pulmonary infections represented the most common severe AE (2.8%), resulting in three fatalities in the abemaciclib cohort. However, these trials did not establish a statistically significant association between infection risk and lymphopenia severity (7,8). In the present case, the patient developed grade 3 lymphopenia, grade 1 neutropenia and anemia during abemaciclib therapy, suggestive of myelosuppression. Neutropenia was mild and not considered the primary etiology for PJP, and the endocrine agents letrozole and goserelin administered concurrently have no documented association with lymphopenia in prior studies (18,19). Consequently, abemaciclib-induced lymphopenia was postulated as the critical factor predisposing to PJP. Furthermore, despite the negative HIV serology and absence of pre-existing immunosuppressive comorbidities, the patient exhibited a marked reduction in lymphocyte counts from 1.5 G/l pre-treatment to 0.5 G/l (August 2023), with CD4+ T-cell counts of 244 cells/µl during abemaciclib therapy (Fig. 3). Such profound lymphopenia likely induced immunosuppression, heightening susceptibility to Pneumocystis infection.

Drug-induced interstitial lung disease (ILD) is indeed a recognized AE of abemaciclib, and this possibility was thoroughly considered in the present case. However, there are distinct differences between ILD and PJP. Drug-induced ILD typically manifests with low-grade fever (20), whereas the patient in the present case exhibited a high fever of 39.6°C. Moreover, chest CT revealed focal consolidation, whereas ILD typically presents with bilateral symmetrical ground-glass opacities on imaging (21). The patient also showed rapid clinical improvement following administration of TMP/SMX, whereas ILD-related changes usually resolve more gradually, often requiring corticosteroids (22). Based on these findings, particularly the focal consolidation on CT, high fever and dramatic response to anti-PJP therapy, a diagnosis of PJP infection was strongly favored over drug-induced ILD in the present case.

Diagnostic modalities for PJP include serum BG, PCR, Gomori methenamine silver (GMS) staining and mNGS. In the present case, serum BG was negative, and a negative serum BG result may reflect insufficient β-glucan release during early-stage infection (<48 h) (23). Whilst repeat testing during the symptomatic peak is feasible, even a positive result cannot specify the causative pathogen. Thus, mNGS was performed for pathogen identification. mNGS is an untargeted, broad-spectrum pathogen detection platform wherein total DNA/RNA from clinical specimens undergoes high-throughput sequencing, generating pathogen taxonomic data through alignment with reference databases. This enables rapid, unbiased pathogen identification without prior targeting. A single sequencing run can identify diverse pathogens, including viruses, bacteria, fungi and parasites (24). Recent studies have reported the diagnostic performance of mNGS for PJP, with sensitivity and specificity of 92.3 and 87.4%, respectively (25,26). Whilst mNGS exhibits comparable sensitivity with PCR, it demonstrates superiority in detecting co-infections. Moreover, although combined GMS staining and mNGS enhances diagnostic accuracy to 96.2% (27), empiric TMP/SMX therapy was promptly initiated in the present case, given the risk of clinical deterioration during the diagnostic interval, leading to rapid symptomatic resolution. Moreover, to avoid incurring additional costs, GMS and PCR were not performed.

Lymphopenia, akin to neutropenia, compromises immune function, rendering the host susceptible to bacterial, viral and other pathogenic invasions. This elevates the risk of infections, potentially culminating in fatal outcomes. Whilst standardized approaches exist for detecting and managing abemaciclib-induced neutropenia (28), evidence-based protocols for identifying and mitigating abemaciclib-associated lymphopenia remain deficient. Currently, there are no established protocols for discontinuing treatment, adjusting doses or offering symptomatic care after lymphopenia, thereby notably elevating the risk of opportunistic infections such as PJP.

In conclusion, the present case demonstrates that lymphopenia is a common and clinically significant AE associated with abemaciclib therapy. Therefore, routine surveillance of absolute lymphocyte counts (ALC) and CD4+ T-cell subsets is warranted during abemaciclib treatment. PJP prophylaxis should be considered in patients exhibiting persistent lymphopenia (ALC, <0.5×109/l) or CD4+ T-cell depletion (<300 cells/µl), particularly when concomitant risk factors exist, including concomitant immunosuppressive agents or pre-existing immunodeficiencies. The preferred regimen is oral TMP/SMX (160/800 mg) once daily; for patients intolerant to TMP/SMX, oral dapsone (100 mg once daily or 50 mg twice daily) may be administered as an alternative. Clinicians should monitor for hematologic toxicities associated with TMP/SMX and dapsone, both of which are associated with a risk of neutropenia (29,30). Co-administration with abemaciclib may amplify the risk and severity of neutropenia, necessitating vigilant hematologic surveillance. Additionally, abemaciclib is predominantly metabolized by CYP3A4, whereas TMP/SMX is primarily metabolized via CYP2C9/CYP2C19, and dapsone via CYP2E1/CYP2C. Consequently, no dose adjustment of abemaciclib is required when used concomitantly with these medications (31–33).

Supplementary Material

Supporting Data

Acknowledgements

Not applicable.

Funding

The present study was supported by the Science and Technology Commission of Shanghai Municipality (grant no. 20Z21900300).

Availability of data and materials

The mNGS data generated in the present study may be found in the figshare database under accession number 29591006 or at the following URL: https://figshare.com/s/88124d28658523f8f3f9. The personal information of the patient is anonymized for privacy protection.

Authors' contributions

WH and SL was involved in the study's conceptualization, methodology, validation, data curation and writing - original draft preparation. YQ participated in the study's conceptualization and contributed by performing patient follow-up, writing-reviewing and editing and supervision. CS was involved in data acquisition. CW and JB were involved in patient follow-up. All authors read and approved the final version of the manuscript. WH, SL, YQ, CS, CW and JB confirm the authenticity of all raw data.

Ethics approval and consent to participate

Not applicable.

Patient consent for publication

Written and signed consent was obtained from the patient to publish their history, images, clinical data and other data included in the present manuscript.

Competing interests

The authors declare that they have no competing interests.

References