Letermovir Treatment for Refractory or Resistant Cytomegalovirus Infection or Disease or with Concurrent Organ Dysfunction: A Phase 2 Open Label Study

Abstract

Introduction:

Refractory or resistant cytomegalovirus (CMV) infection and disease pose a significant challenge in immunocompromised patients, including solid organ transplant (SOT) and allogeneic hematopoietic cell transplant (allo-HCT) recipients. This study aimed to evaluate letermovir as a treatment for patients with CMV infection or disease.

Methods:

We performed an open-label, phase II non-randomized clinical trial. Adult and paediatric (≥12 years of age) patients who had undergone an SOT or allo-HCT and who required antiviral treatment for refractory or resistant CMV or who had CMV with concurrent organ dysfunction were eligible. Patients received letermovir treatment daily for up to 12 weeks with an optional additional 12 weeks of therapy for secondary prophylaxis if clinically indicated. The primary objectives were to evaluate the safety and efficacy of letermovir treatment based upon virological and clinical responses.

Results:

Ten patients were enrolled in the study, and seven patients completed the study treatment and follow-up period. The overall virological response (defined as a complete virological response at the end of the study period) rate was 60% in the study population. The study drug was well tolerated, as only two patients experienced study drug-related toxicity and only one grade 3 toxicity (elevated ALT) was observed. Letermovir was not associated with acute kidney injury, hepatotoxicity, cardiac arrhythmias, or bone marrow suppression.

Conclusion:

In this limited sample, letermovir for CMV treatment was safe and well tolerated. Further research is needed to determine if letermovir can be used for the treatment of refractory or resistant CMV infection or disease.

Introduction

Cytomegalovirus (CMV) is a ubiquitous double-stranded DNA virus belonging to the Herpesviridae family and is endemic in all human populations (1). Seroprevalence studies have shown that CMV infects over 50% of individuals 30 years of age or older (2), with geographical location and socioeconomic status playing important roles in the virus’ prevalence (2).

After primary acquisition, CMV establishes a latent infection that is usually suppressed by the host's immune system (3). However, the virus can reactivate in immunocompromised individuals, such as those undergoing solid organ transplantation (SOT) or allogeneic hematopoietic cell transplantation (allo-HCT) (4). CMV replication can cause significant end-organ damage in transplant recipients (5,6); it can also promote adverse immunological effects, including graft-versus-host disease or graft rejection (7,8). Despite the availability of effective antivirals, CMV remains an important cause of morbidity and mortality post transplantation (9,10). Its management is further complicated by the risk of drug-drug interactions or adverse events with existing therapies, as well as the risk of antiviral drug resistance.

Letermovir is a relatively novel antiviral agent that was approved by the U.S. Food and Drug Administration (FDA) in 2017 for the prophylaxis of CMV in high-risk allo-HCT recipients. It was also approved for prophylaxis of CMV in high-risk kidney transplant recipients in 2023 (11,12). Compared to existing antivirals, letermovir has a unique mechanism of action that inhibits the viral terminase complex (13). Its safety and efficacy profile have been demonstrated in several phase I and II trials as well as two pivotal phase III studies evaluating letermovir prophylaxis among CMV seropositive allo-HCT recipients (14) and kidney transplant recipients (15). The allo-HCT study also included 70 patients who had detectable CMV DNA at the time of randomization (14). Although this subgroup was not included in the primary analysis of the trial, patients who received letermovir with relatively low levels of detectable CMV at baseline were less likely to develop clinically significant CMV infection compared to those who received placebo (16). To further explore this finding, we performed a proof-of-concept, open-label phase II trial using letermovir for the treatment of CMV in patients with refractory or resistant CMV infection or disease and in those with severe baseline organ dysfunction.

Methods

Trial design

We conducted the Study Of Letermovir in refrActory or resistant CytomEgalovirus infection or disease (SOLACE), a proof-of-concept, single-arm, open-label phase II trial. We evaluated the use of letermovir treatment for CMV infection or disease in immunocompromised patients. Patients also required a documented CMV infection that was refractory or resistant to FDA-approved treatments available at that time or baseline organ dysfunction, including myelosuppression or renal dysfunction, for which treatment with available antiviral agents would pose a greater risk for further organ dysfunction. We selected these populations as they represented a group of patients with a high unmet medical need that could potentially benefit from a novel antiviral.

The protocol was designed by the study investigators without input from the funding agency. The trial reflects the standard of care and clinical practice at the study sites in the management of patients with CMV infection or disease during the study period. The study was conducted at the Dana-Farber Cancer Institute (DFCI), the Boston Children's Hospital, Massachusetts General Hospital, and the Brigham and Women's Hospital, in Boston, Massachusetts. The Office for Human Research Studies of the DFCI approved the study protocol. This manuscript is reported according to the Consolidated Standards for Reporting of Trials (CONSORT) (17).

Participants

Adult and paediatric (≥12 years of age) patients who had undergone SOT or allo-HCT and who had required antiviral treatment for CMV were considered eligible for study inclusion. Other immunocompromised patients were also eligible (see Supplementary Material). Participants needed to weigh at least 30 kg and have clinically significant CMV infection, defined as either documented CMV disease (18) or persistent CMV viremia (CMV viral load (VL) ≥500 IU/mL on consecutive measurements obtained at least one day apart). CMV disease was defined as the presence of an appropriate clinical syndrome with documentation of CMV in tissue from the relevant organ or CMV syndrome. Refractory CMV disease and infection were defined as ≥14 days of standard CMV treatment without clinical improvement, and failure to achieve >1 log₁₀ reduction in CMV VL after ≥14 days of standard treatment (19), respectively. Participants also required evidence of a refractory CMV infection, a CMV isolate that was resistant to standard antiviral therapy, or significant end-organ dysfunction, which would preclude the usage of a safe and effective alternate antiviral agent (see Supplemental Table 1). Specific eligibility criteria are described in Supplemental Table 1.

Patients were excluded if they had: (i) a history of allergic reactions attributed to compounds of chemical or biological composition similar to letermovir, (ii) Child-Pugh class C severe hepatic insufficiency at screening, or (iii) a life expectancy of less than a week as determined by their primary treatment physician. Please see the Supplemental Materials for a complete list of inclusion and exclusion criteria.

Interventions

Patients received letermovir treatment daily for up to 12 weeks with an optional additional 12 weeks of treatment for secondary prophylaxis if clinically indicated at the treatment provider's discretion. Treatment was administered on an in-patient or out-patient basis. Dosing for letermovir was 480 mg PO or IV daily for patients not receiving concomitant cyclosporine. The dose was decreased to 240 mg daily for patients receiving concomitant cyclosporine. A second dose on day 1 of treatment (12 hours apart) was administered as a loading dose to reach steady state drug levels faster. No other dose modifications were allowed. No investigational or commercial agents with the intent to treat the participant's CMV infection or disease were permitted. Upon treatment discontinuation, patients were followed for an additional 12 weeks to monitor for reactivation. During the study period, patients had CMV VL measured on a weekly basis. The assay was performed at BWH Clinical Laboratories (quantifiable range of the assay, 137–9,100,000 IU/mL).

Study objectives

The primary objectives were to evaluate the efficacy and safety of letermovir treatment for patients with CMV infection or disease and for whom treatment with available antiviral therapy during the study period either was ineffective or was likely to cause drug toxicities.

Treatment efficacy was evaluated using both virological and clinical responses. Virological response was defined as: (i) a decrease in CMV DNA levels from baseline, measured at week 2 from treatment onset, and (ii) ≥2 log decreased from baseline, or an undetectable CMV VL, measured at week 5. A complete virological response was defined as two consecutive CMV DNA levels <137 IU/mL and the overall virological response was calculated as the proportion of patients who achieved a complete virological response at the end of the study period. Clinical response was defined as: (i) stabilization of clinical disease by week 3 (ie, no worsening signs or symptoms compared to week 1) and (ii) improvement of clinical disease by week 6.

Patients were considered to have experienced virological failure if (i) the CMV DNA viral load was unchanged or higher from baseline when measured on week 2; (ii) the decline in CMV DNA was <2 log on week 5; or (iii) they experienced CMV recurrence while on letermovir therapy. CMV recurrence was defined as plasma CMV DNA ≥137 IU/mL in ≥2 consecutive samples in a patient who had previously achieved a plasma CMV DNA <137 IU/mL.

Treatment safety was measured by capturing any Common Terminology Criteria for Adverse Events (CTCAE) grade 3 (20) or higher adverse event without an alternative explanation. Secondary objectives included describing any potential emergence of letermovir-resistant CMV isolates. CMV-specific T cell responses were also measured in a random selection of participants as an exploratory outcome. Specimens were collected in a longitudinal fashion and the assay was performed in batches by Eurofins-Viracor at the end of the enrollment period.

Sample size

The study was a two-stage design accruing 12 patients into the first stage and 20 additional patients into the second stage if 5 or more patients responded in the first stage. Based on data from previous studies involving the treatment of refractory or resistant CMV infection (19), the response rate of 60% or higher would be considered efficacious and 35% or lower inefficacious. With this hypothesis, the study would provide over 90% power with below 10% type I error rate.

Statistical methods

The final dataset was reviewed for errors in data entry and missing values. No participant had incomplete data for the outcomes of interest. Participants were analyzed in an intention-to-treat (ITT) analysis. Baseline patient characteristics, efficacy, and toxicity outcomes were presented descriptively. All statistical analyses were performed using SAS 9.3 (SAS Institute Inc., Cary, NC), and R v2.13.2 (the CRAN project).

Funding source

The study was funded by Merck through the Merck Study Investigator Program. Merck had no role in study design, data analysis, interpretation, or reporting of the study findings.

Results

Participants

Between October 2018 and February 2022, 163 patients were assessed for study participation; ultimately, 10 patients were enrolled in the study as most were ineligible (see Figure 1). The study was discontinued prematurely due to challenges in patient recruitment, including the FDA approval of maribavir for the treatment of refractory or resistant CMV. All 10 participants started letermovir, but only 6 completed the treatment plan. Of the four who did not complete the study course, three died from complications of their underlying disease as determined by the patient's treating team and one discontinued the study product due to intractable diarrhea.

Figure 1: Study participant eligibility assessment, enrollment, and completion.

Patient characteristics are presented in Table 1. The median age at study enrollment was 62 years (range, 52–77 years) and five participants (50%) were female. Nine patients were enrolled with CMV viremia, and one patient had both CMV viremia and disease. The main underlying conditions predisposing study participants to CMV infection and disease were SOT (n = 5), allo-HCT (n = 3), and lymphoma (n = 2).

Table 1:

Baseline characteristics of study population

	No. (%); N = 10
Age
≥65	5 (10)
Median (range)	61.5 (52–77)
Sex
Female	5 (50)
Male	5 (50)
Race
White	7 (70)
Black	2 (20)
Asian	1 (10)
Underlying disease
ALL	1 (10)
MDS	2 (20)
Lung transplant	2 (20)
Kidney transplant	3 (30)
Lymphoma	1 (10)
MALT lymphoma	1 (10)
Main risk for CMV infection
Hematopoietic cell transplant	3 (30)
Lung transplant	2 (20)
Kidney transplant	3 (30)
MALT of the parotids	1 (10)
Immunocompromised (lymphoma)	1 (10)
CMV entry criteria
CMV infection only	9 (90)
Both infection and disease	1 (10)
Refractory
Yes	6 (60)
Myelosuppression criteria
Yes	9 (90)
Renal dysfunction
Yes	8 (80)
Prior CMV antiviral treatment
Valganciclovir	8 (80)
Ganciclovir	7 (70)
Foscarnet	3 (30)
Number of prior CMV antiviral treatments,†	2.5 (1–4)
median (range)

*Unless otherwise specified

^†Several patients received the same antiviral agent multiple times; these counted as separate prior treatments

ALL = Acute lymphoblastic leukemia; CMV = Cytomegalovirus; MDS = Myelodysplastic syndrome; MALT = Mucosa-associated lymphoid tissue lymphoma

Primary outcome

Patient outcomes are reported in Table 2 and CMV VLs among responders and non-responders are presented in Table 3. The overall response rate was 60%, with five patients responding virologically within the first six weeks of therapy and one additional patient responding afterward. The median CMV VL on study entry was 2200 IU/mL (range, 657–6744 IU/mL). The median treatment duration was 47 days (interquartile range [IQR], 11–90 days) and the median time to first negative CMV VL was 13 days (IQR, 9–180].

Table 2:

Patient outcomes

Patient ID	Age	Sex assigned at birth	Underlying condition	CMV serostatus*	CMV disease	Time from transplant or diagnosis†	Initial CMV VL	Virological response at week 2	Virological response at week 5	Overall virological response	Clinical response at week 3	Clinical response at week 6	Completed the study
001	54	Female	SOT – Lung	D+/R-	Yes	6	6744	No	No	No	No	No	No
002	57	Male	Allo-HCT - ALL	D+/R+	No	13	3501	Yes	No	Yes	-	-	Yes
003	52	Female	SOT - Kidney	X/R-	No	182	2195	Yes	Yes	Yes	-	-	Yes
004	65	Male	Allo-HCT - MDS	D-/R+	No	7	623	No	No	No	-	-	No
005	69	Male	Allo-HCT - MDS	D-/R+	No	4	1934	No	No	No	-	-	No
006	67	Male	SOT - Lung	D+/R-	No	22	3389	Yes	Yes	Yes	-	-	Yes
007	56	Male	SOT - Kidney	D+/R-	No	18	981	Yes	Yes	Yes	-	-	Yes
008	70	Female	MALT lymphoma - Parotids	N/A	No	46	3476	Yes	Yes	Yes	-	-	No
009	77	Female	Lymphoma	N/A	No	0	2204	No	Yes	Yes	-	-	Yes
010	58	Female	SOT - Kidney	D+/R-	No	6	776	No	No	No	-	-	Yes

^*As applicable for transplant recipients

^†Time from transplant or underlying disease diagnosis to study participation in months

Notes: Patient ID 001 discontinued the study product due to intractable diarrhea. Patients ID 004, 005, and 008 died during the study period

D = Donor; R = Recipient; X = Serostatus unknown; SOT = Solid organ transplantation; HCT = Hematopoietic stem cell transplant; ALL = Acute lymphoblastic leukemia; MDS = Myelodysplastic syndrome; MALT = Mucosa-assisted lymphoid tissue

Table 3:

Cytomegalovirus viral load over time

	CMV vral load over time, IU/mL
	Non-responder				Responder				All
Timepoint	n	Med	Min	Max	n	Med	Min	Max	N	Med	Min	Max
Baseline	4	1380	657	6744	6	2797	981	3501	10	2200	657	6744
Within 2 weeks	4	2910	1380	4124	6	549	130	5660	10	1673	130	5660
Between 2 and 6 weeks					6	87	0	1064
Between 6 and 12 weeks					6	81	0	522
Off treatment or post-treatment					5	33	0	584

Med = Median; Min = Minimum; Max = Maximum

Of the entire study population, six patients completed the study. Among these participants, one participant who did not have a complete virological response was unwilling to comply with the required oral medication regimen. Among the participants who did not complete the study follow-up period, one participant achieved documented CMV clearance before transitioning to palliative care and remained with an undetectable CMV VL until death.

Secondary outcomes

Letermovir was well tolerated and only two patients experienced study drug-related toxicity (Table 4). One patient (patient ID 3) experienced grade 1 nausea during the first week of the treatment, which was resolved shortly. The other patient (patient ID 7) experienced grade 3 elevation of alanine aminotransferase, grade 2 elevation of aspartate aminotransferase, grade 1 myalgia, and grade 1 elevation of alkaline phosphatase during the first week of the study treatment.

Table 4:

Letermovir-related toxicity

	Letermovir-related
	toxicity grade
	1	2	3
CTCAE v5.0	N	N	N
Alanine aminotransferase increased			1
Alkaline phosphatase increased	1
Aspartate aminotransferase increased		1
Myalgia	1
Nausea	1

CTCAE = Common Terminology Criteria for Adverse Events

Viral kinetics differed significantly between patients and are presented in Figure 2. Two patients (patient ID 1 and patient ID 10) were classified as non-responders due to their failure to achieve an appropriate clinical and virological response at week 3 of treatment, respectively. These patients were transitioned to ganciclovir and valganciclovir respectively, as they had improved bone marrow function. Both patients subsequently achieved undetectable CMV viral loads. Among seven participants who had a sample sent for antiviral resistance testing, including three without a complete virological response, no participant was found to have a viral isolate with letermovir resistance-associated mutations.

Figure 2: Viral kinetics.

CMV-specific T cell responses were measured in six participants, with serial measurements available in five participants. All six participants exhibited a significant CD8⁺ T cell response at all time points evaluated (defined as CMV-specific CD8 responses >0.2% when whole blood is stimulated with pp65). In contrast, only three participants exhibited a significant CD4⁺ T cell response (defined as CMV-specific CD4 responses >0.2% when whole blood is stimulated with CMV lysate) while on treatment. There were insufficient data to stratify results among participants who did and did not respond to therapy. Due to the low number of measurements, no significant trends were observed with regards to the magnitude of CMV-specific T cell responses during the course of therapy.

Discussion

In this open-label, single-arm, phase II trial, our objective was to evaluate the safety and efficacy of letermovir as a treatment for patients with CMV infection or for whom antiviral therapy was either unlikely to be effective or may incur important drug toxicities. We observed an overall response rate of 60% in the study population. While the observed response rate suggests that letermovir has some therapeutic potential when initiated at relatively low CMV viral loads, the sample size was insufficient to evaluate the primary efficacy endpoint with significant power.

The heterogeneity in viral kinetics was another noteworthy finding, with four participants clearing their CMV viremia between the second and fifth week of treatment. This variability suggests that factors beyond the choice of antiviral medication may influence the course of CMV viremia clearance. Patient-specific elements such as the net degree of immune suppression and drug compliance could contribute to these differences, underscoring the necessity for individualized patient care and regular monitoring of CMV DNA levels.

Previous studies have demonstrated that letermovir has a relatively lower genetic barrier to resistance compared to other CMV antivirals (21). However, our study did not observe any participants developing a viral isolate with letermovir resistance–conferring mutations, likely because participants were not exposed to letermovir for prolonged periods of time with rising viral loads. These results are consistent with recent findings from a large prophylactic randomized controlled trial evaluating letermovir prophylaxis after kidney transplantation, in which no resistance mutations were observed in patients with breakthrough viremia (15). Nevertheless, this finding should be interpreted with caution as our sample size was small. Larger, more comprehensive studies would be needed to accurately assess the risk of developing antiviral resistance with letermovir treatment.

From a safety perspective, the majority of reported adverse events were not attributed to letermovir. The drug was also not linked with episodes of acute kidney injury, hepatotoxicity, cardiac arrhythmias, or bone marrow suppression, thus emphasizing its relative safety profile within this specific population. This is a critical aspect to consider, given that patients in this population often suffer from significant end-organ dysfunction, including renal failure and bone marrow suppression.

Several limitations to this study must be highlighted. First, the study design lacked a control group, rendering a direct comparison with the current standard of care impossible. Further, the non-randomized nature of our trial might have led to selection bias, although we attempted to mitigate this in the study design with careful consideration of inclusion and exclusion criteria. Second, the small sample size limits our statistical power and restricts the generalizability of our findings. In addition, we employed strict definitions for virological and clinical response to report reliable findings. Third, the limited duration of follow-up may have impacted the accurate assessment of the long-term efficacy of letermovir. Fourth, the median VL at study onset was relatively low, which precludes generalizability to patients with higher levels of CMV DNA levels. Lastly, the vulnerable status of the patient population, underscored by the death of three enrolled patients due to complications unrelated to letermovir, adds an additional layer of complexity to data interpretation.

In conclusion, our study provides evidence for the potential use of letermovir in patients with refractory or resistant CMV infection with relatively lower CMV viral loads. Given its remarkable safety profile, consideration may be given to evaluating its use in combination with another CMV antiviral for the treatment of severe or complicated cases at higher risk of virological failure (22).

Funding Statement

This research was supported by Merck through the Merck Study Investigator Program.

Contributors:

Conceptualization, MP Cheng, K Chen, S Burchett, E Moulton, M Desjardins, LR Baden, S Koo, AR Letourneau, SP Hammond, J Ritz, R Soiffer, NC Issa, HT Kim, AC Sherman; Data Curation, MP Cheng, IH Gonzalez-Bocco, E Arbonna-Haddad, M Aleissa, K Chen, E Zhou, K Beluch, A Cho, M Desjardins, LR Baden, S Koo, AR Letourneau, SP Hammond, HT Kim, AC Sherman; Funding Acquisition, LR Baden, S Koo, AR Letourneau, SP Hammond, AC Sherman; Formal Analysis, IH Gonzalez-Bocco, E Arbonna-Haddad, M Aleissa, K Chen, E Zhou, K Beluch, A Cho, LR Baden, S Koo, AR Letourneau, SP Hammond, HT Kim, AC Sherman; Resources, S Koo; Supervision, MP Cheng, J Ritz, HT Kim, AC Sherman; Visualization, HT Kim; Project Administration, MP Cheng, IH Gonzalez-Bocco, E Arbonna-Haddad, M Aleissa, K Chen, E Zhou, K Beluch, A Cho, E Moulton, LR Baden, S Koo, AR Letourneau, SP Hammond, J Ritz, R Soiffer, NC Issa, AC Sherman; Investigation, MP Cheng, IH Gonzalez-Bocco, K Chen, E Zhou, A Cho, S Burchett, E Moulton, LR Baden, S Koo, AR Letourneau, SP Hammond, R Soiffer, NC Issa, HT Kim, AC Sherman; Methodology, MP Cheng, E Arbonna-Haddad, K Chen, K Beluch, A Cho, E Moulton, LR Baden, S Koo, AR Letourneau, SP Hammond, J Ritz, HT Kim, AC Sherman; Writing – Original Draft, MP Cheng, IH Gonzalez-Bocco, AC Sherman; Writing – Review & Editing, MP Cheng, IH Gonzalez-Bocco, E Arbonna-Haddad, M Aleissa, K Chen, E Zhou, K Beluch, A Cho, S Burchett, E Moulton, M Desjardins, LR Baden, S Koo, AR Letourneau, SP Hammond, J Ritz, R Soiffer, NC Issa, HT Kim, AC Sherman.

Ethics Approval:

The Dana-Farber Cancer Institute review board approved this study.

Informed Consent:

The authors confirm that informed patient consent was obtained.

Registry and the Registration No. of the Study/Trial:

ClinicalTrials.gov number: NCT03728426

Data Accessibility:

All data will not be made publicly available. Researchers who require access to the study data can contact the corresponding author for further information.

Funding:

This research was supported by Merck through the Merck Study Investigator Program.

Disclosures:

MP Cheng reports research support from the Canadian Institutes of Health Research and is supported by the Fonds de Recherche du Québec – Santé; research contracts from Cidara Therapeutics, Scynexis, and Amplyx Pharmaceuticals; honoraria from AstraZeneca, Takeda, Merck, and Pfizer; three pending patents (Methods for detecting tissue damage, graft-versus-host disease, and infections using cell-free DNA profiling; Methods for assessing the severity and progression of SARS-CoV-2 infections using cell-free DNA; and Rapid identification of antimicrobial resistance and other microbial phenotypes using highly-multiplexed fluorescence in situ hybridization); stock options as a member of the scientific advisory board for GEn1E Lifesciences and Nomic Bio; and equity as co-founder of Kanvas Biosciences. SP Hammond has research funding from Cidara, F2G, GSK, and Scynexis and has served as a consultant or on an advisory board for F2G, Pfizer and Seres Therapeutics. The other authors have nothing to disclose.

Peer Review:

This manuscript has been peer reviewed.

Animal Studies:

N/A