Human Tissue Distribution of Anidulafungin and Micafungin

ABSTRACT

Concentrations of anidulafungin and micafungin were determined in eight different tissues obtained during autopsy of four deceased individuals who had been treated with anidulafungin and of seven who had received micafungin. The largest amounts were recovered from liver, with anidulafungin concentrations of 11.01 to 66.50 μg/g and micafungin levels of 0.36 to 5.53 μg/g (0.65 μg/g 30 days after the last administration). The lowest anidulafungin levels were measured in skeletal muscle, and the lowest micafungin concentrations were in kidneys.

TEXT

The echinocandins anidulafungin and micafungin are recommended for treatment of invasive candidiasis, because they are highly effective against candidemia (1–5). However, knowledge of their concentrations in various human tissues is scarce (6, 7). Therefore, we quantified anidulafungin and micafungin concentrations in eight tissues of deceased adults.

(Some of the data in this study were presented at the 25th Scientific Symposium of the Austrian Pharmacological Society [Innsbruck, Austria; 2019].)

The study protocol was approved by the local ethics committee. Tissue samples were obtained during routine autopsies of patients who had been treated with anidulafungin or micafungin within 30 days prior to death and who on admission had granted permission for scientific use of residual specimens. Between death and autopsy, the bodies were kept at 4°C for up to 85 h. Anidulafungin and micafungin concentrations were quantified in thyroid, lung, myocardium, spleen, liver, kidney, pancreas, and skeletal muscle. The echinocandins were extracted and quantified as described previously for brain (8). In brief, 0.2 g of tissue was homogenized with acetonitrile and methanol, and concentrations were measured with high-performance liquid chromatography and UV detection (8, 9). The external standards consisted of animal tissue spiked with anidulafungin or micafungin. A lower limit of quantification (LLOQ) of 0.10 μg/g was achieved for anidulafungin in all tissues but skeletal muscle (LLOQ = 0.05 μg/g) and for micafungin in all tissues but myocardium, kidney, and pancreas (LLOQ = 0.20 μg/g). The process efficiency amounted to 40% to 80%. Postmortem stability of echinocandins was assessed by incubation of patient tissue at 4°C for 96 h. Anidulafungin was stable in all tissues and micafungin in all tissues but spleen, where the concentration declined by 39% within 96 h.

For statistical calculations, we used IBM (Armonk, NY, USA) SPSS Statistics software version 26.0. The significance of the differences between anidulafungin and micafungin concentrations was analyzed by Mann-Whitney U test. Correlations between anidulafungin and micafungin concentrations and body weight, daily dose, cumulative dose, interval between the last administration and death, and interval from death to autopsy were assessed using linear regression.

Tissue samples were obtained from four females (median [range], 69 [45 to 74] years old) who had been treated with anidulafungin and from two females and five males (58 [38 to 76] years old) who had received micafungin. Whereas median cumulative doses were similar in both treatment groups, the median interval between the last administration and death was much shorter in the anidulafungin group than in the micafungin group, where it was longer than 100 h in most patients. This might explain the significantly higher tissue concentrations of anidulafungin. Nevertheless, anidulafungin tissue concentrations in patient 2, who died 290 h after the last infusion (cumulative dose, 1,800 mg), exceeded micafungin tissue concentrations of patient 6, who died 230 h after the last administration (cumulative dose, 5,300 mg). Linear regression did not reveal any correlation between the anidulafungin and micafungin concentrations and the analyzed demographic characteristics and treatment parameters.

The largest amounts of anidulafungin were recovered from the liver, followed by spleen, lung, and pancreas (median > 10 μg/g) (Table 1). In kidney, thyroid, myocardium, and skeletal muscle, anidulafungin concentrations were lower (median <10 μg/g) (Table 1). The highest micafungin concentrations were measured in liver, thyroid, and lung (median > 0.25 μg/g) (Table 2). Lower concentrations were found in myocardium, skeletal muscle, and kidney (median < 0.20 μg/g) (Table 2). Twelve days after the last administration, anidulafungin was detected in all tissues of patient 2, and concentrations between 0.51 and 11.01 μg/g were determined. Notably, in the liver of patient 5, micafungin could be measured even 30 days after the last administration (0.65 μg/g).

TABLE 1.

Patient characteristics and anidulafungin concentrations in autopsy samples^a

Characteristic	Patient no.				Median (range)
	1	2	3	4
Wt (kg)	135	85	43	50	67.50 (43–135)
Cumulative dose (mg/kg)	5.19	21.18	34.88	24.00	22.59 (5.19–34.88)
Treatment duration (days)	6	7	14	11	9 (6–14)
Interval between last AFG administration and death (h)	32	299	11.50	29	30.50 (11.50–299)
Interval between death and sampling (h)	14	11	15	9	12.50 (9–15)
Total interval last AFG administration and sampling (h)	46	310	26.50	38	42.00 (26.50–310)
Main diagnoses	COPD, pneumonia	DLBCL, septic shock	DLBCL, st. p. HSCT, ileus, pneumonia	Sepsis, peritonitis after sigmoid perforation
Indication for AFG therapy	Empiric treatment	Invasive candidiasis (candidemia)	Invasive aspergillosis (AFG treatment in combination with voriconazole)	Invasive candidiasis (candidemia)
Anidulafungin concn (μg/g) in:
Thyroid	2.10	1.02	9.74	12.75	5.92 (1.02–12.75)
Lung	3.56	5.08	28.76	20.31	12.69 (3.56–28.76)
Myocardium	1.21	0.98	8.17	5.25	3.23 (0.98–8.17)
Spleen	6.16	2.98	36.68	24.43	15.30 (2.98–36.68)
Liver	17.33	11.01	58.17	66.50	37.75 (11.01–66.50)
Kidney	2.72	1.25	10.23	12.67	6.47 (1.25–12.67)
Pancreas	4.08	1.97	18.92	16.77	10.43 (1.97–18.92)
Skeletal muscleb	1.14	0.51	NA	7.70	1.14 (0.51–7.70)

^aAFG, anidulafungin; COPD, chronic obstructive pulmonary disease; DLBCL, diffuse large B-cell lymphoma; st. p., status post; HSCT, hematopoietic stem cell transplantation; NA, not available (sampling missed).

^bRectus abdominis muscle; lower limit of quantification for thyroid, lung, myocardium, spleen, liver, kidney, and pancreas is 0.10 μg/g, and that for skeletal muscle is 0.05 μg/g.

TABLE 2.

Patient characteristics and micafungin concentrations in autopsy samples^a

Characteristic	Patient no.							Median (range)
	5	6	7	8	9	10	11
Wt (kg)	80	92	80	60	50	81	85	80 (50–92)
Cumulative dose (mg/kg)	13.75	57.61	22.50	8.33	27.00	3.70	47.06	22.50 (3.70–57.61)
Treatment duration (days)	19	28	18	4	9	3	39	18 (3–39)
Interval between last MFG administration and death (h)	712	230	110	24	148	324	235	230 (24–712)
Interval between death and sampling (h)	58	85	38	74	20	80	29	58 (20–85)
Total interval last MFG administration and sampling (h)	770	315	148	98	168	404	264	264 (98–770)
Main diagnoses	Burkitt-lymphoma relapse, st. p. HSCT	Wound infection (C. albicans), septic shock, osteomyelofibrosis	Cholangiocarcinoma, biliary-pleural fistula, sepsis	St. p. LuTX, ischemic stroke, pneumonia	T-cell lymphoma, cardiogenic shock	Fungal endophthalmitis, pneumonia, COPD	St. p. LTX, hepatic artery occlusion, wound infection
Indication for MFG therapy	Empiric treatment	Invasive candidiasis (candidemia)	Invasive candidiasis (pleural empyema)	Empiric treatment	Empiric treatment	Invasive candidiasis (Candida endophthalmitis)	Suspected Candida cholangitis
Micafungin concn (μg/g) in:
Thyroid	<0.10	0.43	0.47	0.44	<0.10	<0.10	0.44	0.43 (<0.10–0.47)
Lung	<0.10	0.20	0.70	1.81	0.23	0.18	0.29	0.26 (<0.10–1.81)
Myocardium	<0.20	<0.20	0.45	0.44	<0.20	<0.20	0.24	0.19 (<0.20–0.45)
Spleen	<0.10	0.13	0.73	1.44	<0.10	<0.10	0.31	0.22 (<0.10–1.44)
Liver	0.65	3.14	5.53	2.57	0.36	0.36	3.31	2.86 (0.36–5.53)
Kidney	<0.20	<0.20	0.50	0.77	<0.20	<0.20	<0.20	0.13 (<0.20–0.77)
Pancreas	<0.20	<0.20	0.46	0.26	<0.20	<0.20	0.44	0.15 (<0.20–0.46)
Skeletal muscleb	<0.10	1.78	0.18	0.13	<0.10	<0.10	0.18	0.16 (<0.10–1.78)

^aMFG, micafungin; st. p., status post; HSCT, hematopoietic stem cell transplantation; C. albicans, Candida albicans; LuTX, lung transplantation; COPD, chronic obstructive pulmonary disease; LTX, liver transplantation.

^bRectus abdominis muscle; lower limit of quantification for thyroid, lung, spleen, liver and skeletal muscle is 0.10 μg/g and for myocardium, kidney and pancreas 0.20 μg/g.

Invasive candidiasis may affect various tissues, causing deep-seated candidiasis. Anidulafungin tissue concentrations approached or even exceeded therapeutic plasma levels (6). For our study population, however, no plasma levels were available.

MICs of anidulafungin and micafungin range from 0.008 to 2.0 μg/ml for most pathogenic Candida isolates (10). Anidulafungin tissue concentrations exceeded these values in almost all tissue samples.

The limited size and the heterogeneity of our study population are considerable limitations. We cannot rule out agonal or postmortem changes of echinocandin tissue concentrations. However, after a 96-h incubation at 4°C, concentrations remained unchanged in all tissues but spleen. Nevertheless, patients 6 and 10 presented comparatively low micafungin concentrations in their tissue samples, which were taken 85 h and 80 h after death, respectively. Furthermore, echinocandin extraction from autopsy samples might be slightly less effective than extraction from calibrators. In rats treated with [¹⁴C]anidulafungin, however, tissue concentrations were comparable (11). Anidulafungin and micafungin display plasma protein binding of 99.0% and 99.9%, respectively. Their binding to protein in tissue is unknown, but protein binding influences echinocandin activity in vitro (12, 13). Autopsy samples comprise different tissue compartments, such as extracellular matrix, various cells, and blood vessels. With our method, we could not detect echinocandins on a cellular or subcellular level.

In brain samples taken from our study population, we had measured anidulafungin concentrations of 0.21 to 2.34 μg/g and micafungin concentrations of 0.18 to 2.88 μg/g. These data were reported previously, together with concentrations in cerebrospinal fluid of critically ill patients, which were even lower than brain concentrations (8). Preclinical studies revealed anidulafungin and micafungin tissue concentrations and distribution comparable with our findings, but in rodents, the highest concentrations were measured in lung (11, 14–16). Clinical studies of bronchoalveolar lavage demonstrated echinocandin accumulation in alveolar macrophages (17–20). Anidulafungin concentrations in ascites fluid and in pleural effusion exceeded those in cerebrospinal fluid but were below plasma levels and the MICs for some pathogenic Candida strains (21). Human tissue distribution of amphotericin B displays a pattern similar to that of anidulafungin and micafungin, with accumulation in liver and spleen (22–24). Fluconazole and voriconazole appear to be distributed more homogeneously over different tissues (25, 26).

In conclusion, anidulafungin and micafungin reach high concentrations in liver, spleen, and lung, whereas concentrations in myocardium and skeletal muscle are low. Evaluation of the clinical impact of anidulafungin and micafungin target site penetration warrants clinical outcome studies.