The amount of injected ¹⁷⁷Lu-octreotate strongly influences biodistribution and dosimetry in C57BL/6N mice

Abstract

Background. ¹⁷⁷Lu-octreotate therapy has proven to give favorable results after treatment of patients with neuroendocrine tumors. Much focus has been on the binding and uptake of ¹⁷⁷Lu-octreotate in tumor tissue, but biodistribution properties in normal tissues is still not fully understood, and the effect of receptor saturation may be important. The aim of this study was to investigate the influence of the amount of ¹⁷⁷Lu-octreotate on the biodistribution of ¹⁷⁷Lu-octreotate in normal tissues in mice.

Material and methods. C57BL/6N female mice were intravenously injected with 0.1–150 MBq ¹⁷⁷Lu-octreotate (0.039 μg peptide/MBq). The mice were killed 0.25 h to 14 days after injection by cardiac puncture under anesthesia. Activity concentration was determined in blood, bone marrow, kidneys, liver, lungs, pancreas, and spleen, and mean absorbed doses were calculated.

Results. The activity concentration varied with time and amount of injected activity. At 4–8 h after injection, a local maximum in activity concentration was found for liver, lungs, pancreas, and spleen. With the exception for the lower injected activities (0.1–1 MBq), the overall highest uptake was found in the kidneys (%IA/g). Large variations were found and the activity concentration in kidneys was 11–23%IA/g at 4 h, and 0.22–1.9%IA/g at 7 days after injection. Furthermore, a clear reduction in activity concentration with increased injected activity was observed for lungs, pancreas and spleen.

Conclusion. The activity concentration in all tissues investigated was strongly influenced by the amount of ¹⁷⁷Lu-octreotate injected. Large differences in mean absorbed dose per unit injected activity were found between low (0.1–1 MBq, 0.0039–0.039 μg) and moderate amounts (5–45 MBq, 0.2–1.8 μg). Furthermore, the results clearly showed the need for better ways to estimate absorbed dose to bone marrow other than methods based on a single blood sample analysis. Since the absorbed dose to critical organs will limit the amount of ¹⁷⁷Lu-octreotate administered, these findings must be taken into consideration when optimizing this type of therapy.

The localization of neuroendocrine (NE) tumors is today routinely done by scintigraphy using radiolabeled somatostatin analogues [1,2]. The method relies on a high expression of somatostatin receptors (SSTRs) by many of these tumor types, and a high affinity of radiolabeled somatostatin analogs to these receptors [3,4]. There are five different isoforms of human SSTRs, where different NE tumor types and individual tumors display differences in expression of each of these SSTR subtypes [3,4].

The somatostatin analog ¹⁷⁷Lu-octreotate, with highest affinity to SSTR2 and SSTR5, is used for therapy with promising results [5]. In patients, ¹⁷⁷Lu-octreotate treatment has resulted in tumor regression, increased overall survival, and an improvement in quality of life [5–7]. However, despite the fact that some organs have high normal expression of SSTRs, the kidneys are the main dose limiting organ in this type of therapy. The accumulation of ¹⁷⁷Lu-octreotate in the kidneys is at least partly due to reabsorption in the proximal tubular cells, but SSTR-mediated and peritubular uptake may also play a role [8]. The reabsorption process in the kidneys is due to several transport mechanisms, e.g. receptor mediated endocytosis (megalin/cubilin receptors and SSTRs), amino acid/oligopeptide transporters, pinocytosis, and passive diffusion [9].

Thus, this treatment modality is limited by uptake and resulting absorbed dose to normal tissues, e.g. kidney and bone marrow [1,9]. Saturation effects of SSTRs due to high amounts of ¹⁷⁷Lu-octreotate administered needs to be considered in the treatment planning, both for tumor and normal tissues [9]. Saturation effects have previously been suggested from studies with radiolabeled somatostatin analogs in tumor-bearing nude mice (BALB/c) in organs with high uptake, such as tumor, lungs, adrenals, spleen, and liver [10,11]. Saturation in tumor tissue has also been discussed in treatment of patients [12,13].

The previously published studies on the biodistribution of ¹⁷⁷Lu-octreotate in mice have, to our knowledge, been performed on immune-deficient BALB/c mice transplanted with tumor tissue [10,11,14–16]. However, the overall response related to both internal and external stimulus is strongly related to the immune response [17–19]. Thus, investigation of normal mice with a retained immune system related to both molecular response and biodistribution is of interest, and the response might very well be dependent on the immune function of the mice. Furthermore, the biokinetics at early time-points has not been studied in detail, and it can be assumed that the activity concentration in the organs reaches a maximum well before 1–4 h after injection, time-points commonly used in present studies.

The aim of this study was to investigate the biodistribution of ¹⁷⁷Lu-octreotate in normal C57BL/6N mice versus time after injection and the influence of amount of ¹⁷⁷Lu-octreotate injected.

Material and methods

Radiopharmaceuticals

¹⁷⁷LuCl₃ and DOTA-Tyr³-octreotate were acquired from I.D.B. Holland BV (Baarle-Nassau, The Netherlands). Preparation and radiolabeling of DOTA-Tyr³-octreotate with ¹⁷⁷LuCl₃ were conducted according to the instructions of the manufacturer. The specific activity of ¹⁷⁷Lu-octreotate was 25.7 MBq/μg. The ¹⁷⁷Lu-octreotate stock solution (10.3 GBq/ml, 370 μg/ml) was diluted with saline solution to final activity concentrations. Instant thin layer chromatography (ITLC) was used for quality control, and fraction of peptide bound ¹⁷⁷Lu was > 99%.

To determine the injected activity in each animal, the ¹⁷⁷Lu activity in the syringes was measured with a calibrated well-type ionization chamber (CRC-15R; Capintec, IA, USA) before and after administration.

Biodistribution of¹⁷⁷Lu-octreotate

Ten-week-old female C57BL/6N mice (Taconic, Hudson, USA) were i.v. injected with ¹⁷⁷Lu- octreotate in the tail vein. The animals were killed by cardiac puncture under anesthesia with pentobarbitalnatrium (APL, Sweden) and samples of blood and bone marrow, together with kidneys, liver, lungs, pancreas, and spleen were collected. Bone marrow was acquired from the femurs. The tissue samples were weighed and the ¹⁷⁷Lu activity was measured with a gamma counter (Wallac 1480 Wizard, Wallac Oy, Turku, Finland). The efficiency of the gamma counter was calibrated towards the efficiency of the ionization chamber. Corrections were done for background and dead time. The experimental protocol was approved by the local ethics committee for Animal Research in Gothenburg, Sweden.

Influence of time

Nine groups (n = 4/group) were i.v. injected with 15 (SD = 0.5) MBq ¹⁷⁷Lu-octreotate, corresponding to 0.59 μg peptide. The mice were killed at 0.25 h, 0.5 h, 1 h, 4 h, 8 h, 24 h, 3 days, 7 days, and 14 days after injection and the organs and tissues were collected and measured as described above.

Influence of amount of ¹⁷⁷Lu-octreotate administered

Eighteen groups (n = 4/group) were i.v. injected with 0.1 (SD = 0.0075), 1 (0.050), 5 (0.21), 45 (1.2), 90 (2.9), and 150 (3.0) MBq ¹⁷⁷Lu-octreotate, corresponding to 0.0039, 0.039, 0.20, 1.8, 3.5, and 5.9 μg peptide. The animals were killed at 4 h, 24 h, and 7 days after injection and the organs and tissues were collected and measured as described above. In the analysis of these groups, the data points for 15 MBq at 4 h, 24 h, and 7 days from the time dependency investigation was included.

Dosimetry

The ¹⁷⁷Lu activity concentration in the investigated organs and tissues was calculated as the fraction of injected activity per unit mass and the tissue-to-blood activity concentration ratio (Ti/B = C_tissue(t)/C_blood(t)) was also determined.

The mean absorbed dose to tissues from ¹⁷⁷Lu-octreotate was estimated using the Medical International Radiation Dose (MIRD) formalism [20]. The value of the self-absorbed fraction was set to 1, and the cross-absorbed fraction was set to 0 for all organs. Only the contribution from electrons were included in the dosimetric calculations (n_iE_i = 147 keV/decay). The exclusion of the photon contribution to the absorbed dose would only marginally influence the absorbed dose due to the low yield of the emitted photons [21]. The cumulated activity was determined from the measured activity concentrations for each tissue, and was calculated by fitting mono- or bi-exponential curves to the measured data. Only self-dose was calculated.

Results

The biodistribution after injection of 15 MBq ¹⁷⁷Lu-octreotate was determined for the time-points 0.25 h–14 days (Table I). The highest activity concentration for blood, kidneys, liver, lungs, pancreas and spleen was found at the first time-point measured (0.25 h after injection). For blood and kidneys the activity concentration then decreased monotonously with time, while for the other organs studied, with the exception for the bone marrow, local maxima in activity concentration was observed at 4 h and 8 h after injection. The activity concentration decreased rapidly between 0.25 h and 1 h after injection in blood (95% reduction), and in liver, lungs, and spleen (87–94% reduction).

Table I. Influence of time on measured activity concentration. The activity concentration (given as%IA/g) in various tissue types at 0.25 h– 14 days after i.v. injection of 15 MBq ¹⁷⁷Lu-octreotate (0.59 μg) in C57BL/6N mice. Data are given as mean (SEM), (n = 4/group).

Time after injection	Activity concentration (%IA/g)
Tissue	Blood	Bone marrow	Kidneys	Liver	Lungs	Pancreas	Spleen
0.25 h	9.0 (0.4)	5.2 (2.1)	53 (4)	2.6 (0.3)	9.2 (0.6)	6.9 (0.5)	3.5 (0.3)
0.5 h	3.5 (0.4)	3.6 (1.3)	39 (3)	1.3 (0.1)	5.7 (0.2)	6.6 (0.7)	1.7 (0.04)
1 h	0.42 (0.11)	7.5 (0.6)	26 (2)	0.24 (0.02)	1.2 (0.2)	2.2 (0.3)	0.20 (0.02)
4 h	0.23 (0.05)	1.9 (0.4)	16 (0.5)	0.42 (0.01)	1.5 (0.06)	3.7 (0.2)	0.45 (0.14)
8 h	0.049 (0.008)	0.76 (0.09)	11 (0.8)	0.86 (0.13)	1.4 (0.08)	2.2 (0.2)	0.29 (0.06)
24 h	0.024 (0.005)	4.0 (2.5)	7.2 (0.2)	0.15 (0.01)	0.82 (0.03)	0.46 (0.03)	0.15 (0.01)
3 days	0.0044 (0.0004)	0.84 (0.12)	2.8 (0.2)	0.093 (0.006)	0.41 (0.03)	0.14 (0.008)	0.086 (0.002)
7 days	0.0018 (0.0004)	0.074 (0.018)	1.0 (0.04)	0.058 (0.006)	0.16 (0.002)	0.042 (0.005)	0.060 (0.007)
14 days	0.00069 (0.00026)	0.035 (0.006)	0.36 (0.03)	0.050 (0.002)	0.067 (0.006)	0.024 (0.001)	0.053 (0.005)

The tissue-to-blood activity concentration ratios (Ti/B) generally increased with time after injection (Figure 1). The kidneys showed the highest Ti/B values at all time-points with mean values between 5.8 (0.25 h) and 650 (3 days). The lowest Ti/B values were obtained for pancreas and spleen, with similar levels for bone marrow at most time-points and very high uncertainty of the data points with very high mean values.

Figure 1. The Ti/B activity concentration ratio versus time after injection of 15 MBq ¹⁷⁷Lu-octreotate (0.59 μg octreotate) for (A) bone marrow, pancreas, and spleen and (B) kidneys, liver, and lungs. Error bars represent SEM. Note the logarithmic scale of the x-axes and the difference in scale of the y-axes.

The decrease in ¹⁷⁷Lu-concentration with time in the studied organs varied with amount of injected activity without a clear dose-response relationship (Figure 2). In the blood, the concentration (%IA/g) decreased to 10–34% at 24 h compared to at 4 h, while the values at 7 days stabilized with a range between 3% and 8%. The liver, kidneys, and spleen all had a decrease between 26% and 59% at 24 h, while at 7 days, the decrease was 9–42%, 2–10%, and 4–46%, respectively. Pancreas showed a decrease in concentration with values between 8% and 30% at 24 h and 0.5% and 4% at 7 days. The bone marrow showed the largest interval with values between 5% and 200% at 24 h, and 3–62% at 7 days.

Figure 2. The ¹⁷⁷Lu activity concentration, C_tissue, in the different tissues versus time after injection (4–168 h). Data are given for injected activities of 0.1–150 MBq ¹⁷⁷Lu-octreotate (0.0039–5.9 μg). Error bars represent SEM. Note the logarithmic scale of the x-axis.

The influence of amount of ¹⁷⁷Lu-octreotate administered (0.1–150 MBq) on the biodistribution is shown in Figure 3. The highest concentration, given as%IA/g, was found for the lowest amount administered (0.1 MBq), with the exception for the kidneys. In general, the activity concentration decreased with increased amount of ¹⁷⁷Lu-octreotate injected. Bone marrow showed the largest relative difference (100% between 0.1 MBq and 90 MBq groups) at 4 h after injection. At both 24 h and 7 days after injection, the lungs showed the largest relative reduction in activity concentration between the 0.1 MBq and 150 MBq groups (19–0.14%IA/g at 24 h and 3.8–0.044%IA/g at 7 days). In kidneys, the activity concentration increased with higher amount of ¹⁷⁷Lu-octreotate injected. The highest relative increase, 350%, was found at 168 h for 5 MBq compared with 0.1 MBq. The liver was found to have the most consistent change in %IA/g with a 50–80% reduction at all time-points and at all injected activities, except at 7 days after injection.

Figure 3. The ¹⁷⁷Lu activity concentration, C_tissue, for the different tissues versus injected activity of ¹⁷⁷Lu-octreotate (0.1–150 MBq, 0.0039–5.9 μg). Data are given for the time points 4 h, 24 h, and 7 days. Error bars represent SEM. Note the logarithmic scale.

The Ti/B value for bone marrow, lungs, and pancreas generally decreased with increasing injected activities, while for kidneys, liver, and spleen the reversed was observed (Supplementary Figure 1, available online at http://informahealthcare.com/doi/abs/10.3109/0284186X.2015.1027001). The lungs showed a relatively strong reduction with increased injected activity between 0.1 and 5 MBq with a subsequent stabilization. Concerning the same interval, the kidneys and liver showed only a slight dependence with injected activity, followed by a strong increase in Ti/B values between 5 and 150 MBq for all time-points studied.

Dosimetric calculations were carried out for all groups (Tables II and III). In the groups that received 15 MBq ¹⁷⁷Lu-octreotate, the absorbed dose per injected activity was highest for kidneys, followed by bone marrow for late time-points (Table II). Most of the absorbed dose was given during the first 7 days after injection for 15 MBq IA (Table II). Despite the fast decrease in activity concentration in the organs, the dosimetric estimations showed that 6–9% of the absorbed dose in lungs, spleen, pancreas and liver received during the first two weeks, was given during the first hour (Table II).

Table II. Influence of time on absorbed dose. Estimated mean absorbed dose per injected activity (mGy/MBq) in the investigated tissues at different time-points after injection of 15 MBq ¹⁷⁷Lu-octreotate (0.59 μg), and also extrapolated to infinity (t = ∞). Data are given as mean (n = 4/group).

Time after injection	Mean absorbed dose per injected activity (mGy/MBq)
	Blood	Bone marrow	Kidney	Liver	Lung	Pancreas	Spleen
0.25 h	0.84	0.49	4.8	1.5	0.9	0.48	0.34
0.5 h	2.1	1.7	14	1.9	2.4	1.7	0.9
1 h	2.7	3.8	28	2.2	3.7	3.9	1.2
4 h	2.9	12	72	3.2	6.9	14	1.3
8 h	2.9	18	120	4.3	11	22	1.4
24 h	2.9	23	230	8.1	24	32	2.0
3 days	3.3	110	380	12	42	41	5.5
7 days	3.4	120	470	15	55	44	8.9
14 days	3.5	130	500	18	61	46	12
∞	3.5	150	510	20	62	47	14

Table III. Influence of amount of injected activity on absorbed dose. Estimated mean absorbed dose per injected activity (mGy/MBq) in the investigated tissues after injection of 0.1–150 MBq ¹⁷⁷Lu-octreotate (0.0039–5.9 μg). Estimations were based on biodistribution data from 4 h, 24 h, and 7 days after injection and extrapolated to infinity. Data are given as mean (n = 4/group).

Injected Activity (MBq)	Mean absorbed dose per injected activity (mGy/MBq)
	Blood	Bone marrow	Kidney	Liver	Lung	Pancreas	Spleen
0.1	9.5	1700	400	43	1500	670	120
1	3.6	150	310	17	350	280	26
5	3.7	49	700	47	150	130	20
15	3.1	240	530	16	63	55	16
45	0.73	13	520	16	27	25	12
90	0.58	24	440	14	19	12	13
150	0.45	11	360	14	15	13	11

The lowest values were obtained for blood, spleen, and liver, except for very early time-points. In general the absorbed dose per injected activity decreased with increasing amount of injected ¹⁷⁷Lu-octreotate, but aberrant results were found for several tissue types (Table III). Overall, the kidneys received the highest absorbed dose, where the highest mean absorbed dose per injected activity was 700 mGy/MBq at 5 MBq, with a subsequent decrease for lower and higher activities administered. However, after the 0.1 MBq injection, the estimated absorbed dose per injected activity in both bone marrow and the lungs was higher, with values of 1500–1700 mGy/MBq.

Discussion

In this study, the biodistribution of various amounts of ¹⁷⁷Lu-octreotate in C57BL/6N mice was investigated at several time-points after i.v. injection. The relation between biodistribution of radiopharmaceuticals after diagnostic and therapeutic amounts is of great importance, especially for therapy, since it is the basis for dosimetry and therapy planning for potential risk organs. The studied organs and tissues included blood, bone marrow, kidneys, liver, lungs, pancreas, and spleen, where special emphasis on bone marrow and kidney was taken due to the dose limiting nature of these organs in ¹⁷⁷Lu-octreotate therapy of neuroendocrine tumors. The time-points were chosen to give a detailed account of the biodistribution. In the dependence of injected activity on the biodistribution, the time-points 4 h, 24 h and 7 days were chosen as both early and late time-points are vital for proper absorbed dose estimations. A large dependence of amount of injected ¹⁷⁷Lu-octreotate was observed and the results clearly demonstrate the importance of studying the biodistribution both at very early and very late time-points.

In the absorbed dose calculations in the present study, a homogeneous activity distribution in the organs was assumed. However, it is known that radionuclide distribution in, e.g. the kidneys are heterogeneous, and rodent studies have shown differences in uptake in the kidney cortical tissue than medulla 24 h after injection [22,23]. Previous studies in rats have shown a concentration at 24 h in the outer medulla of at least 50–60% of the cortical activity [22]. In contrast, in female C57BL mice, the highest renal uptake was observed in the outer medulla, with lower uptake in the cortex [23]. Furthermore, in the absorbed dose calculations, only the self-dose was estimated and the absorbed fraction was set to unity. The absorbed fractions have previously been estimated to be higher than 86% for the investigated organs, with the exception for bone marrow with 74% [24]. However, due to inhomogeneous activity distributions within the organs, the absorbed fraction may vary. In addition, the cross doses were not included in the calculations. However, the cross doses can be assumed to only marginally contribute to the absorbed dose due to the short mean range of the electrons emitted by ¹⁷⁷Lu (0.67 mm in water).

The highest ¹⁷⁷Lu uptake in most of the investigated tissues was obtained already at 15 min after injection. As this was the first time-point investigated, maximum uptake could potentially occur either earlier or between the measured time-points of 0.25 h and 0.5 h. The high value at this early time-point can in part be explained by the high amount of blood in these organs together with a high activity concentration in blood early after injection. However, this would assume a relatively constant Ti/B activity concentration ratio between the tissues investigated at this early time-point. Instead, at 15 min after injection, the Ti/B ratio varied between 0.3 and 5.8 depending on tissue. Many of these organs (liver, lungs, pancreas, and spleen) also had a local maximum in activity concentration at 4 h and/or 8 h after injection. The reason for this behavior is unknown but could be due to several factors, such as recycling of the low amount of SSTRs in the cell, another uptake/retention mechanism, e.g. involvement of transport proteins, or degradation of the ¹⁷⁷Lu-octreotate conjugate. However, more research is needed to verify and clarify this behavior, both in animals and man.

All organs, except the kidneys, showed a decreased concentration when the injected activity increased from 0.1 to 1 MBq (0.0039–0.039 μg), and bone marrow and lungs showed the largest relative reduction. In the kidneys, rather an increase in uptake could be detected when the injected activity increased. However, it is important to note that the absolute uptake (amount of activity in the organ) still increased with the amount of injected ¹⁷⁷Lu-octreotate. The dependence of amount of injected peptide has previously been studied in GOT1 tumor bearing nude mice 24 h after ¹¹¹In-octreotide administration (0.001–25 μg) [11]. In that study, the relative uptake by the liver did not depend on amount of injected peptide (0.12%IA/g, 0.001–25 μg), which is in contrast to the present study, where the relative uptake decreased from 0.41%IA/g (0.0039 μg) to 0.08%IA/g (5.9 μg). Furthermore, in that study a decrease in activity concentration was shown in lungs and spleen at levels higher than 0.1 μg, while in the present study, a clear dose dependent decrease in activity concentration could be seen already at the lower amounts injected (0.0039–0.039 μg).

Other studies have investigated the biodistribution of ¹⁷⁷Lu-octreotate with various results (Supplementary Table I, available online at http://informahealthcare.com/doi/abs/10.3109/0284186X.2015.1027001) [10,14,15,25]. Those studies were performed on BALB/c nude mice transplanted with neuroendocrine tumor tissue. Concerning similar amounts of peptide injected, Kölby et al. (0.25 μg, GOT1 tumor) found lower uptake values 24 h after injection in the kidneys compared to the present study (0.2 μg, no tumor) (6–8%IA/g vs. 9.1%IA/g) [25]. Also Dalmo et al. (0.2 μg, GOT2 tumor) found lower values 24 h after injection with uptake values 50% lower compared to the present study [10]. Similar numbers were found also for blood, liver, and spleen, while in pancreas a 40% higher uptake was found in the previous study [10]. Furthermore, a faster clearance was found for all tissues compared to the present study. Schmitt et al. (0.7 μg, H69 tumor) also observed lower uptake values with a faster clearance compared to the present study (0.6 μg), with the exception for liver and blood [14]. However, de Araujo et al. (1 μg, AR42J tumor) found much higher uptake values compared to the present study with 2–3 times higher values in blood, liver, pancreas, and spleen 24 h after injection [15]. Only the kidneys showed a lower uptake value than the present study with a 40% reduction. The reason for the large variations between the studies is unknown. The main differences between those studies and the present one are the presence of neuroendocrine tumor and the use of immunodeficient mice. However, the tumor uptake varied considerably between the studies without apparent effects on the biodistribution in the normal tissues. Thus, endocrine and/or other physiological effects due to the presence of different neuroendocine tumor tissues are more probable explanations to these discrepancies, together with the different strains of mice used.

The dependence of the amount of injected peptide has also been studied in man. The dependence on peptide amount on the resulting absorbed dose in various organs was studied in one patient receiving therapeutic amounts of ¹¹¹In-octreotide [13]. A lower peptide amount (20 μg) was shown to give a higher absorbed dose per unit injected activity compared with a higher peptide amount (40 μg, 0.56 mGy/MBq vs. 0.33 mGy/MBq in the kidneys, respectively). Kletting et al. investigated the dependence of amount of injected peptide (octreotate) on the residence times in various organs and tumor, and it was found that the simulated therapeutic residence time following therapeutic amounts of octreotate was greatly increased (three-fold) compared with when measuring with lower pretherapeutic (diagnostic) amounts in the kidneys (amount of peptide not given) [26]. In another study, where the impact of peptide mass on tumor binding of ⁶⁸Ga-DOTATOC was explored, a dependence of total amount of peptide administered was found, with highest tumor-to-normal tissue ratio found for the lowest amount of octreotide administered (50 μg) [27]. However, simply measuring the residence time or uptake at one point in time is not sufficient for accurate absorbed dose assessment. Instead these two parameters need to be investigated in conjunction, due to their dependencies on each other. An accurate dosimetric calculation can only be performed once an understanding of these parameters has been obtained. Furthermore, when optimizing ¹⁷⁷Lu-octreotate therapy the highest tumor-to-normal tissue absorbed dose ratios should be sought for, especially regarding the most important critical organs, kidneys and bone marrow.

An interesting finding in the present study is the strong increase of Ti/B value for bone marrow [bone marrow-to-blood concentration ratio (BM/B)] with time. Bone marrow dosimetry is clinically often based on blood concentration curves [28,29]. Most methods assume a constant BM/B value with time and the blood sample is collected relatively early after administration [30]. The present findings clearly show an increased BM/B value with time (0.6 after 0.25 h and 50 at 14 days). We have previously found BM/B values of 1.7–2.7 in patients 1–5 days after injection of ¹¹¹In-octreotide [31]. If BM/B for ¹⁷⁷Lu-octreotate increases in a similar way in humans as in mice, the methods for bone marrow dosimetry must clearly be revised for ¹⁷⁷Lu-octreotate. The reason for increased BM/B values with time could in theory be due to high SSTR expression in bone marrow, e.g. in activated lymphocytes and/or bone marrow stem cells [30]. Another, probably more realistic reason is accumulation of ¹⁷⁷Lu (metabolically released from octreotate), probably transported via certain transport proteins in the blood, such as transferrin [32]. The relatively slow increase rate favors the second theory, since receptor binding would be much faster.

In conclusion, the present study shows that the biodistribution of ¹⁷⁷Lu strongly depends on the injected amount of ¹⁷⁷Lu-octreotate. The uptake and the mean absorbed dose per unit injected activity in the kidneys showed large differences between low (0.1–1 MBq, 0.0039–0.039 μg) and moderate amounts (5–45 MBq, 0.2–1.8 μg). Similar relationships were found also for bone marrow, lungs, and pancreas. These findings are important when treatment planning is based on the biodistribution of a diagnostic amount or when the amount per fraction is altered during treatment. Furthermore, the influence of amount of injected ¹⁷⁷Lu-octreotate on BM/B values with time demonstrate the need for better ways to estimate absorbed dose to bone marrow other than methods based on a single blood sample analysis.

Supplementary material available online

Supplementary Figure 1 and Table I available online at http://informahealthcare.com/doi/abs/10.3109/0284186X.2015.1027001

Acknowledgments

The authors thank Lilian Karlsson and Ann Wikström for their skilled technical assistance. This study was supported by grants from the Swedish Research Council (grant no. 21073), the Swedish Cancer Society (grant no. 3427), BioCARE – a National Strategic Research Program at the University of Gothenburg, the Swedish Radiation Safety Authority, the King Gustav V Jubilee Clinic Cancer Research Foundation, the Sahlgrenska University Hospital Research Funds, the Assar Gabrielsson Cancer Research Foundation, the Lions Cancerfond Väst, and the Adlerbertska Research Foundation in Gothenburg, Sweden. The authors report no conflicts of interest. The authors alone are responsible for the content and writing of the paper.

Declaration of interest: The authors report no conflicts of interest. The authors alone are responsible for the content and writing of the paper.