Efficacy and safety of lurbinectedin and doxorubicin in relapsed small cell lung cancer. Results from an expansion cohort of a phase I study

Background A phase I study found remarkable activity and manageable toxicity for doxorubicin (bolus) plus lurbinectedin (1-h intravenous [i.v.] infusion) on Day 1 every three weeks (q3wk) as second-line therapy in relapsed small cell lung cancer (SCLC). An expansion cohort further evaluated this combination. Patients and methods Twenty-eight patients with relapsed SCLC after no more than one line of cytotoxic-containing chemotherapy were treated: 18 (64%) with sensitive disease (chemotherapy-free interval [CTFI] ≥90 days) and ten (36%) with resistant disease (CTFI <90 days; including six with refractory disease [CTFI ≤30 days]). Results Ten patients showed confirmed response (overall response rate [ORR] = 36%); median progression-free survival (PFS) = 3.3 months; median overall survival (OS) = 7.9 months. ORR was 50% in sensitive disease (median PFS = 5.7 months; median OS = 11.5 months) and 10% in resistant disease (median PFS = 1.3 months; median OS = 4.6 months). The main toxicity was transient and reversible myelosuppression. Treatment-related non-hematological events (fatigue, nausea, decreased appetite, vomiting, alopecia) were mostly mild or moderate. Conclusion Doxorubicin 40 mg/m2 and lurbinectedin 2.0 mg/m2 on Day 1 q3wk has shown noteworthy activity in relapsed SCLC and a manageable safety profile. The combination is being evaluated as second-line therapy for SCLC in an ongoing, randomized phase III trial. Clinical trial registration www.ClinicalTrials.gov code: NCT01970540. Date of registration: 22 October, 2013.


Introduction
Lurbinectedin is a selective inhibitor of oncogenic transcription that binds preferentially to guanines located in the GCrich regulatory areas of DNA gene promoters [1,2]. By preventing binding of transcription factors to their recognition sequences, the drug inhibits oncogenic transcription and leads to tumor cell apoptosis [3]. Lurbinectedin also affects the tumor microenvironment landscape by inhibiting activated transcription in tumor-associated macrophages [4].
Studies in mice found antitumor activity for lurbinectedin against different models of xenografted human-derived tumors. In the first-in-human (FiH) study, single-agent lurbinectedin showed antitumor activity in patients with advanced solid tumors (pancreatic cancer, soft tissue sarcoma, melanoma) [5]. The combination of doxorubicin and lurbinectedin resulted in significantly stronger antitumor effect compared to either drug alone in mice bearing xenografted small cell lung cancer (SCLC) tumors, with additive and sometimes synergistic effects [6]. This improved activity led to the conduct of a phase I trial to evaluate this combination of doxorubicin and lurbinectedin in selected advanced solid tumors, including relapsed SCLC. The recommended dose (RD) for the combination was defined at doxorubicin 50 mg/m 2 and lurbinectedin 4.0 mg flat dose (FD) on Day 1 every three weeks (q3wk) [7]. Promising antitumor activity was found in second-line SCLC and endometrial cancer during dose escalation. However, 88% of patients treated at this RD had grade 3/4 neutropenia regardless of relationship. As a result, the combination was further evaluated in an expansion cohort at a lower doxorubicin dose to reduce the incidence of potentially severe myelosuppression. In addition, the lurbinectedin dose was transformed to a body surface area (BSA)-based dose following the finding, in a logistic regression analysis of pooled data from phase II trials with singleagent lurbinectedin, that patients with the lowest BSA values could have a greater possibility of developing grade 3/4 thrombocytopenia. This expansion cohort only enrolled patients with advanced SCLC to be treated as second-line therapy and patients with endometrial cancer. Due to the relevance of the antitumor activity observed in this cohort, the results shown here are focused on advanced SCLC patients.

Eligibility criteria
Eligible patients were aged ≥18 years with confirmed SCLC treated with no more than one prior line of cytotoxiccontaining chemotherapy regimen for advanced disease (other than anthracyclines); with documented disease progression during or immediately after last therapy; who had recovered from previous toxicities; ≥3 weeks since last anticancer therapy; Eastern Cooperative Oncology Group performance status ≤2; normal left ventricular ejection fraction (LVEF); and adequate bone marrow, hepatic and renal function, including albumin ≥3.0 g/dL.
Patients were excluded if they had symptomatic progressive or corticosteroid-requiring brain metastases/ leptomeningeal involvement; were pregnant or lactating women, or were not using effective contraception; had prior bone marrow/stem cell transplantation, relevant cardiac disease, alcohol consumption or cirrhosis, active uncontrolled infection, or any disease interfering with study outcome.

Study treatment
Patients were treated with doxorubicin 40 mg/m 2 as bolus followed by lurbinectedin 2.0 mg/m 2 intravenously (i.v.) over one hour on Day 1 q3wk. This dose was based on the RD defined during dose escalation (doxorubicin 50 mg/m 2 and lurbinectedin 4.0 mg flat dose [FD]) [7], with a reduced doxorubicin dose and the lurbinectedin dose transformed to a BSAbased dose. Both drug doses were capped at a BSA of 2.0 m 2 . Patients who received ten cycles of the combination or had to discontinue doxorubicin due to a cardiac adverse event were switched to lurbinectedin alone at its single-agent RD defined during the First-in-Human study (4.0 mg/m 2 on Day 1 q3wk) to prevent doxorubicin-induced cardiomyopathy [5,8].
Commercially available doxorubicin was provided. Lurbinectedin was supplied as a lyophilized powder concentrate, reconstituted in sterile water for injection, and diluted with glucose 5% or sodium chloride 0.9% solution. All patients received standard antiemetic prophylaxis before each infusion. Treatment was given until disease progression, unacceptable toxicity, intercurrent illness precluding study continuation, patient refusal and/or non-compliance with study requirements, treatment delay >15 days (except if clear clinical benefit), and requirement of >2 dose reductions.

Dose-limiting toxicities
Protocol guidelines concerning dose feasibility and definition of dose-limiting toxicities (DLTs) that were used during dose escalation in this study also applied to this cohort [7]. The regimen of doxorubicin 40 mg/m 2 and lurbinectedin 2.0 mg/ m 2 on Day 1 q3wk would be considered feasible if less than one third of evaluable patients had DLTs during Cycle 1.

Study assessments
Hematology and biochemistry tests were conducted at baseline, weekly during Cycle 1, and before each lurbinectedin infusion and on Day 10 during subsequent cycles. Electrocardiograms and LVEF assessments were done at baseline, and were repeated at doxorubicin discontinuation or if clinically indicated.
Antitumor activity was evaluated every two cycles according to the Response Evaluation Criteria In Solid Tumors (RECIST) v.1.1 [9]. Overall response rate (ORR) was the percentage of patients with complete (CR) or partial response (PR), and disease control rate (DCR) was the percentage of patients with response or stable disease (SD). Time-to-event parameters were duration of response (DoR), progression-free survival (PFS) and overall survival (OS).
Adverse events (AEs) and laboratory abnormalities were graded using the National Cancer Institute Common Terminology Criteria for Adverse Events (NCI-CTCAE) v.4 [10], and coded with the Medical Dictionary for Regulatory Activities (MedDRA) v.14.1.

Statistical analysis
Continuous variables were presented with summary statistics and categorical variables in frequency tables. Time-to-event variables were calculated using Kaplan-Meier approach. Binomial exact distribution was used to calculate 95% confidence intervals (95%CIs) for categorical variables.
All patients received prior systemic anticancer therapy with platinum compounds and etoposide. Twenty-two patients (79%) received prior radiotherapy, including 11 patients (39%) who were given prophylactic cranial irradiation (PCI).

Efficacy
All treated patients were evaluable for efficacy.  Table 2).
Tumor shrinkage was observed in 18 of 26 (69%) patients with at least one radiological tumor assessment ( Fig. 1): 13 of 18 (72%) with sensitive disease and five of eight (63%) with resistant disease. Antitumor activity achieved with the combination was maintained in four of six patients treated with single-agent lurbinectedin after doxorubicin discontinuation.

Safety
All treated patients were evaluable for safety. The most frequent treatment-related AEs or with unknown relationship were fatigue, nausea, decreased appetite, vomiting, and alopecia ( Table 3). Most of these AEs were grade 1/2, with the most common grade ≥ 3 AEs being fatigue (25%), febrile neutropenia (14%) and nausea (7%). The only treatmentrelated AE to reach grade 4 was febrile neutropenia, in two patients. One episode of grade 5 treatment-related neutropenic infection occurred in one patient with ongoing diabetes, atrial fibrillation and chronic obstructive pulmonary disease, and disease involvement in the brain and lungs. No treatment-related cardiac adverse events associated with the LVEF occurred.
Regardless of relationship, most laboratory abnormalities were grade 1/2. Grade ≥ 3 hematological abnormalities comprised anemia, neutropenia (grade 4 episodes lasting a median o f 3 d a y s [ r a n g e , 1 -6 d a y s ] ) , l e u k o p e n i a , a n d  Table 3). The most common biochemical abnormalities were creatinine and transaminase increases. Grade ≥ 3 biochemical abnormalities consisted of transaminase increases in one patient with normal levels and no liver metastases at baseline, and grade 4 creatinine increase in one patient that was concomitant with severe renal failure unrelated to treatment. Sixteen patients (57%) required granulocyte colonystimulating factor support, seven (25%) were given red blood cell transfusions, and three (11%) received platelet transfusions. Eighteen dosing delays, seven lurbinectedin dose reductions and one doxorubicin dose reduction were the result of treatment-related AEs. Twenty-two patients died during the study, mostly (n = 21) due to disease progression and one due to treatment-related neutropenic infection.

Discussion
A regimen of doxorubicin 40 mg/m 2 and lurbinectedin 2.0 mg/m 2 on Day 1 q3wk showed noteworthy antitumor activity in patients with relapsed SCLC included in an expansion cohort of a phase 1 study evaluating the doxorubicin/ lurbinectedin combination in advanced solid tumors. ORR was 36%, with median DoR 5.2 months, median PFS 3.3 months and median OS 7.9 months. Most responses and longer median time-to-event parameters were found among patients with sensitive disease (ORR = 50%, DoR 5.5 months, PFS 5.7 months, OS 11.5 months). In contrast, patients with resistant disease only had one response (ORR = 10%, DoR 1.8 months) and shorter median survival times (PFS 1.3 months, OS 4.6 months). Of note, all responses occurred among patients with non-refractory disease (ORR = 46%, PFS 5.1 months, OS 10.2 months). Treatment and survival of patients with SCLC has not changed substantially over the last two decades. Response rates to first-line therapy are high, but recurrence is frequent, especially in patients with extensive-stage disease [11,12]. Topotecan is the only second-line therapy approved in the U.S. for patients with sensitive SCLC [12]. However, use of topotecan is challenging because of associated hematological toxicity and modest clinical benefit (ORR = 5-24%; median OS 6-8 months) [13][14][15][16][17][18][19][20]. Similar results have been reported for cyclophosphamide, doxorubicin and vincristine (CAV), a combination also used in second-line treatment of SCLC [13]. Alternative therapies such as monoclonal antibodies (nivolumab, atezolizumab) [21,22] have also been evaluated for relapsed SCLC, but to date none have shown superiority over currently approved therapies.
The results obtained in this study with the doxorubicin/ lurbinectedin combination compare favorably with those reported with second-line therapies for relapsed SCLC. Higher ORRs were achieved with the combination, both at the RD of doxorubicin 50 mg/m 2 and lurbinectedin 4.0 mg FD defined during dose escalation (overall: 65%; sensitive disease: 89%; resistant disease: 38%) [7] and in the expansion cohort at the reduced dose of doxorubicin 40 mg/m 2 and lurbinectedin 2.0 mg/m 2 (overall: 36%; sensitive disease: 50%; resistant disease: 10%). Median survival times were also longer, especially in sensitive disease (PFS 5.8 months at the RD [7]; PFS 5.7 months and OS 11.5 months in the expansion cohort). In the expansion cohort, exclusion of patients with refractory disease (CTFI ≤30 days; a population usually not included in clinical trials due to having a poorer prognosis) increased the ORR to 46% and also resulted in long median survival times (PFS 5.1 months, OS 10.2 months). Of note, results from a cohort of 105 patients treated with single-agent lurbinectedin in a multicenter, multinational phase II Basket trial showed that lurbinectedin is active as second-line therapy in relapsed SCLC (ORR = 35%; median OS 9.3 months). Antitumor activity in this cohort was notable, both in sensitive (ORR = 45%) and resistant disease (ORR = 22%) [23,24]. Of note, 20% of patients treated with single-agent lurbinectedin in this phase II Basket study had refractory disease (CTFI <30 days).
Treatment with the doxorubicin/lurbinectedin combination was generally well tolerated, and was primarily associated with manageable and predictable myelotoxicity. Compared with the RD defined during dose escalation, patients in the expanded cohort showed less grade 4 neutropenia (68% vs. 79%), grade 3 anemia (25% vs. 47%) and treatment-related febrile neutropenia (14% vs. 26%) [7]. Biochemical abnormalities were mostly mild or moderate and asymptomatic, and occurred at similar frequencies in the two cohorts. Some treatment-related non-hematological adverse events (mucositis, 18% vs. 53%; alopecia, 25% vs. 42%) were less common in the expansion cohort than at the RD, while others occurred at similar with CR and nine with PR. Red stars = treatment switch to lurbinectedin alone. CR, complete response; CTFI, chemotherapy-free interval; d, days; PR, partial response frequencies (fatigue, 79% in both cohorts; nausea/ vomiting, 39-68% vs. 58%; decreased appetite, 46% vs. 53%) [7]. No patients in either cohort discontinued treatment with the combination due to treatment-related events. Of note, the absence of treatment-related cardiac events associated with the LVEF either during dose escalation or in the expansion cohort suggested that lurbinectedin does not increase ventricular dysfunction over doxorubicin. The safety profile of single-agent lurbinectedin in the expansion cohort is in agreement with that reported elsewhere [5,23].
In summary, remarkable antitumor activity has been found for a combination of doxorubicin 40 mg/m 2 and lurbinectedin 2.0 mg/m 2 on Day 1 q3wk in an expanded cohort of patients with relapsed SCLC, particularly in sensitive disease, and higher than that reported for currently approved second-line therapies. Compared to this dose, the initial RD defined in this study (doxorubicin 50 mg/m 2 and lurbinectedin 4.0 mg FD) resulted in a higher ORR but also higher toxicity, with more frequent hematological abnormalities and treatment-related mucositis and alopecia. Of note, single-agent lurbinectedin has recently been approved by the U.S. Food and Drug Administration for the treatment of adult patients with metastatic SCLC with disease progression on or after platinum-based chemotherapy. An ongoing open-label, AE, adverse event; ALP, alkaline phosphatase; ALT, alanine aminotransferase; AST, aspartate aminotransferase; NCI-CTCAE, National Cancer Institute Common Terminology Criteria for Adverse Events; q3wk, every three weeks; SCLC, small cell lung cancer randomized phase III trial is evaluating the doxorubicin/ lurbinectedin combination vs. standard-of-care chemotherapy (CAV or topotecan) in patients with SCLC and CTFI ≥30 days that has progressed after one line of platinum-based chemotherapy [25].
Ethics approval The study was approved by the Research Ethics Committees of participating centers in Spain and the U.K. The study was conducted in full accordance with the principles of the World Medical Association Declaration of Helsinki, the International Conference of Harmonisation, and all applicable local guidelines and regulations on good clinical practice.
Consent to participate All patients provided written informed consent to participate in this study.
Consent for publication All patients provided written informed consent about participation in this study and the use of collated data. No individual patient data are included in this article that could be used to identify any individual.
Code availability Not applicable.
Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article's Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article's Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creativecommons.org/licenses/by/4.0/.