The feasibility of targeted imaging and therapy using radiolabeled albumin-binding domain-derived affinity proteins (ADAPTs) has been demonstrated. However, high renal uptake of radioactivity limits the maximum tolerated dose. Successful reduction of renal retention of radiolabeled Fab fragments has been demonstrated by incorporating a cleavable linker between the targeting agent and the radiometal chelator. The present study investigated if the introduction of a glycine-leucine-glycine-lysine (GLGK)-linker would reduce the kidney uptake of radiolabeled ADAPT6 and also compared it with the non-residualizing [125I]I-[(4-hydroxyphenyl)ethyl]maleimide ([125I]I-HPEM) labeling strategy. GLGK was site-specifically coupled to human epidermal growth factor receptor 2 (HER2)-targeting ADAPT6. Conjugates without the cleavable linker were used as controls and all constructs were labeled with lutetium-177 (177Lu). [125I]I-HPEM was coupled to ADAPT6 at the C-terminus. Biodistribution of all constructs was evaluated in NMRI mice 4 h after injection. Specific binding to HER2-expressing cells in vitro was demonstrated for all constructs. No significant difference in kidney uptake was observed between the [177Lu]Lu-2,2 ',2",2"'-(1,4,7,10-tetraazacyclododecane-1,4,7,10-tetrayl)tetraacetic acid-GLGK-conjugates and the controls. The renal activity of [125I]I-HPEM-ADAPT6 was significantly lower compared with all other constructs. In conclusion, the incorporation of the cleavable GLGK-linker did not result in lower renal retention. Therefore, the present study emphasized that, in order to achieve a reduction of renal retention, alternative molecular design strategies may be required for different targeting agents.

Non-invasive radionuclide molecular visualization of human epidermal growth factor receptor type 2 (HER2) can provide stratification of patients for HER2-targeting therapy. This method can also enable monitoring of the response to such therapies, thereby making treatment personalized and more efficient. Clinical evaluation in a phase I study demonstrated that injections of two scaffold protein-based imaging probes, [Tc-99m]Tc-(HE)(3)-G3 and [Tc-99m]Tc-ADAPT6, are safe, well-tolerated and cause a low level of radioactivity in healthy tissue. The goal of this preclinical study was to select the best probe for stratification of patients and response monitoring. Biodistribution of both tracers was compared in mice bearing SKOV-3 xenografts with high HER2 expression or MDA-MB-468 xenografts with very low expression. Changes in accumulation of the probes in SKOV-3 tumors 24 h after injection of trastuzumab were evaluated. Both [Tc-99m]Tc-ADAPT6 and [Tc-99m]Tc-(HE)(3)-G3 permitted high contrast imaging of HER2-expressing tumors and a clear discrimination between tumors with high and low HER2 expression. However, [Tc-99m]Tc-ADAPT6 has better preconditions for higher sensitivity and specificity of stratification. On the other hand, [Tc-99m]Tc-(HE)(3)-G3 is capable of detecting the decrease of HER2 expression on response to trastuzumab therapy only 24 h after injection of the loading dose. This indicates that the [Tc-99m]Tc-(HE)(3)-G3 tracer would be better for monitoring early response to such treatment. The results of this study should be considered in planning of further clinical development of HER2 imaging probes.

Radionuclide molecular imaging of human epidermal growth factor receptor type 2 (HER2) expression may help to stratify breast and gastroesophageal cancer patients for HER2-targeting therapies. Albumin-binding domain-derived affinity proteins (ADAPTs) are a new type of small (46-59 amino acids) protein useful as probes for molecular imaging. The aim of this first-in-humans study was to evaluate the biodistribution, dosimetry, and safety of the HER2-specific Tc-99(m)-ADAPT6. Methods: Twenty-nine patients with primary breast cancer were included. In 22 patients with HER2-positive (n = 11) or HER2-negative (n = 11) histopathology, an intravenous injection of 385 +/- 125 MBq of Tc-99(m)-ADAPT6 was performed, randomized to an injected protein mass of either 500 mu g (n = 11) or 1,000 mu g (n = 11). Planar scintigraphy followed by SPECT imaging was performed after 2, 4, 6, and 24 h. An additional cohort (n = 7) was injected with 165 +/- 29 MBq (injected protein mass, 250 mu g), and imaging was performed after 2 h only. Results: Injections of Tc-99(m)-ADAPT6 were well tolerated at all mass levels and not associated with adverse effects. Tc-99(m)-ADAPT6 cleared rapidly from the blood and most other tissues. The normal organs with the highest accumulation were the kidney, liver, and lung. Effective doses were 0.009 +/- 0.002 and 0.010 +/- 0.003 mSv/MBq for injected protein masses of 500 and 1,000 mu g, respectively. Injection of 500 mu g resulted in excellent discrimination between HER2-positive and HER2-negative tumors as early as 2 h after injection (tumor-to-contralateral breast ratio, 37 +/- 19 vs. 5 +/- 2; P < 0.01). The tumor-to-contralateral breast ratios for HER2-positive tumors were significantly (P < 0.05) higher for an injected mass of 500 mu g than for either 250 or 1,000 mu g. Conclusion: Injections of Tc-99(m)-ADAPT6 are safe and associated with low absorbed and effective doses. A protein dose of 500 mu g is preferable for discrimination between tumors with high and low expression of HER2. Further studies are justified to evaluate whether Tc-99(m)-ADAPT6 can be used as an imaging probe to stratify patients for HER2-targeting therapy in areas where PET imaging is not readily available.

Albumin-binding fusion partners are frequently used as a means for the in vivo half-life extension of small therapeutic molecules that would normally be cleared very rapidly from circulation. However, in applications where small size is key, fusion to an additional molecule can be disadvantageous. Albumin-derived affinity proteins (ADAPTs) are a new type of scaffold proteins based on one of the albumin-binding domains of streptococcal protein G, with engineered binding specificities against numerous targets. Here, we engineered this scaffold further and showed that this domain, as small as 6 kDa, can harbor two distinct binding surfaces and utilize them to interact with two targets simultaneously. These novel ADAPTs were developed to possess affinity toward both serum albumin as well as another clinically relevant target, thus circumventing the need for an albumin-binding fusion partner. To accomplish this, we designed a phage display library and used it to successfully select for single-domain bispecific binders toward a panel of targets: TNFα, prostate-specific antigen (PSA), C-reactive protein (CRP), renin, angiogenin, myeloid-derived growth factor (MYDGF), and insulin. Apart from successfully identifying bispecific binders for all targets, we also demonstrated the formation of the ternary complex consisting of the ADAPT together with albumin and each of the five targets, TNFα, PSA, angiogenin, MYDGF, and insulin. This simultaneous binding of albumin and other targets presents an opportunity to combine the advantages of small molecules with those of larger ones allowing for lower cost of goods and noninvasive administration routes while still maintaining a sufficient in vivo half-life. 

The human epidermal growth factor receptor 2 (HER2) is a clinically validated target for cancer therapy, and targeted therapies are often used in regimens for patients with a high HER2 expression level. Despite the success of current drugs, a number of patients succumb to their disease, which motivates development of novel drugs with other modes of action. We have previously shown that an albumin binding domain-derived affinity protein with specific affinity for HER2, ADAPT(6), can be used to deliver the highly cytotoxic protein domain PE25, a derivative of Pseudomonas exotoxin A, to HER2 overexpressing malignant cells, leading to potent and specific cell killing. In this study we expanded the investigation for an optimal targeting domain and constructed two fusion toxins where a HER2-binding affibody molecule, Z(HER2:2891), or the dual-HER2-binding hybrid Z(HER2:2891)-ADAPT(6) were used for cancer cell targeting. We found that both targeting domains conferred strong binding to HER2; both to the purified extracellular domain and to the HER2 overexpressing cell line SKOV3. This resulted in fusion toxins with high cytotoxic potency toward cell lines with high expression levels of HER2, with EC50 values between 10 and 100 pM. For extension of the plasma half-life, an albumin binding domain was also included. Intravenous injection of the fusion toxins into mice showed a profound influence of the targeting domain on biodistribution. Compared to previous results, with ADAPT(6) as targeting domain, Z(HER2:2891) gave rise to further extension of the plasma half-life and also shifted the clearance route of the fusion toxin from the liver to the kidneys. Collectively, the results show that the targeting domain has a major impact on uptake of PE25-based fusion toxins in different organs. The results also show that PE25-based fusion toxins with high affinity to HER2 do not necessarily increase the cytotoxicity beyond a certain point in affinity. In conclusion, Z(HER2:2891) has the most favorable characteristics as targeting domain for PE25.

Albumin binding domain-Derived Affinity ProTeins (ADAPTs) are small (5 kDa) engineered scaffold proteins that are promising targeting agents for radionuclide-based imaging. A recent clinical study has demonstrated that radiolabeled ADAPTs can efficiently visualize human epidermal growth factor receptor 2 (HER2) expression in breast cancer using SPECT imaging. However, the use of ADAPTs directly labeled with radiometals for targeted radionuclide therapy is limited by their high reabsorption and prolonged retention of activity in kidneys. In this study, we investigated whether a co-injection of lysine or gelofusin, commonly used for reduction of renal uptake of radiolabeled peptides in clinics, would reduce the renal uptake of [Tc-99m]Tc(CO)(3)-ADAPT6 in NMRI mice. In order to better understand the mechanism behind the reabsorption of [Tc-99m]Tc(CO)(3)-ADAPT6, we included several compounds that act on various parts of the reabsorption system in kidneys. Administration of gelofusine, lysine, probenecid, furosemide, mannitol, or colchicine did not change the uptake of [Tc-99m]Tc(CO)(3)-ADAPT6 in kidneys. Sodium maleate reduced the uptake of [Tc-99m]Tc(CO)(3)-ADAPT6 to ca. 25% of the uptake in the control, a high dose of fructose (50 mmol/kg) reduced the uptake by ca. two-fold. However, a lower dose (20 mmol/kg) had no effect. These results indicate that common clinical strategies are not effective for reduction of kidney uptake of [Tc-99m]Tc(CO)(3)-ADAPT6 and that other strategies for reduction of activity uptake or retention in kidneys should be investigated for ADAPT6.

Non-immunoglobulin scaffolds represent a proven group of small affinity proteins that can be engineered in vitro to similar affinity and potency as monoclonal antibodies. Several novel candidate biotherapeutics that exploit the potential advantages scaffold proteins hold over larger and more complex antibodies have been developed over the past decade. The ease of using small and robust binding proteins as flexible and modular building blocks has led to the development of a wide range of innovative approaches to combine them in various bi- and multispecific formats. This progress is expected to aid the ongoing challenge of identifying niche applications where clear differentiation from antibody-based molecules will be key to success. Given the many engineering options that are available for non-immunoglobulin scaffold proteins, they have potential to not only complement but probably also surpass antibodies in certain applications.

ADAPTs are small engineered non-immunoglobulin scaffold proteins, which have demonstrated very promising features as vectors for radionuclide tumour targeting. Radionuclide imaging of human epidermal growth factor 2 (HER2) expression in vivo might be used for stratification of patients for HER2-targeting therapies. ADAPT6, which specifically binds to HER2, has earlier been shown to have very promising features for in vivo targeting of HER2 expressing tumours. In this study we tested the hypothesis that dimerization of ADAPT6 would increase the apparent affinity to HER2 and accordingly improve tumour targeting. To find an optimal molecular design of dimers, a series of ADAPT dimers with different linkers, -SSSG- (DiADAPT6L1), -(SSSG)(2)- (DiADAPT6L2), and -(SSSG)(3)- (DiADAPT6L3) was evaluated. Dimers in combination with optimal linker lengths demonstrated increased apparent affinity to HER2. The best variants, DiADAPT6L2 and DiADAPT6L3 were site-specifically labelled with In-111 and I-125, and compared with a monomeric ADAPT6 in mice bearing HER2-expressing tumours. Despite higher affinity, both dimers had lower tumour uptake and lower tumour-to-organ ratios compared to the monomer. We conclude that improved affinity of a dimeric form of ADAPT does not compensate the disadvantage of increased size. Therefore, increase of affinity should be obtained by affinity maturation and not by dimerization.