Probabilistic modelling of prospective environmental concentrations of gold nanoparticles from medical applications as a basis for risk assessment

Background The use of gold nanoparticles (Au-NP) based medical applications is rising due to their unique physical and chemical properties. Diagnostic devices based on Au-NP are already available in the market or are in clinical trials and Au-NP based therapeutics and theranostics (combined diagnostic and treatment modality) are in the research and development phase. Currently, no information on Au-NP consumption, material flows to and concentrations in the environment are available. Therefore, we estimated prospective maximal consumption of Au-NP from medical applications in the UK and US. We then modelled the Au-NP flows post-use and predicted their environmental concentrations. Furthermore, we assessed the environment risks of Au-NP by comparing the predicted environmental concentrations (PECs) with ecological threshold (PNEC) values. Results The mean annual estimated consumption of Au-NP from medical applications is 540 kg for the UK and 2700 kg for the US. Among the modelled concentrations of Au-NP in environmental compartments, the mean annual PEC of Au-NP in sludge for both the UK and US was estimated at 124 and 145 μg kg−1, respectively. The mean PEC in surface water was estimated at 468 and 4.7 pg L−1, respectively for the UK and US. The NOEC value for the water compartment ranged from 0.12 up to 26,800 μg L−1, with most values in the range of 1000 μg L−1. Conclusion The results using the current set of data indicate that the environmental risk from Au-NP used in nanomedicine in surface waters and from agricultural use of biosolids is minimal in the near future, especially because we have used a worst-case use assessment. More Au-NP toxicity studies are needed for the soil compartment. Electronic supplementary material The online version of this article (doi:10.1186/s12951-015-0150-0) contains supplementary material, which is available to authorized users.


AF1
 It has been assumed that each product by a company for a particular application serves 100% of the market of the US and UK (i.e. no competition) and all patients, irrespective of socioeconomic status etc., have access to these products. For example, when a therapy is in clinical trials for head and neck cancer, we have used the latest publicly available data for number of people diagnosed with head and neck cancer in a particular year and used this data as a prospective population for treatment. Innovative medicines might create excitement with regard to possibility of increasing the life expectancy of a patient; hence we have assumed that all deaths could be prevented if this medicine is used as a last line treatment under the auspices of "expanded access or compassionate use" c . Therefore, mortality figures of people suffering from a particular type of cancer were used. We are aware that not all people will have access to these 'trial' drugs and devices, however, our objective is to model high emission worst case scenario and hence we have included these numbers. Various different disease types and stages of cancer have not been taken into consideration. It is assumed that all patients get treated in the same year, since the model (in the current state of development) doesn't allow for time-based-releases.
 Attempts have been made to reduce risks due to double counting (Exception: There is double counting of two applications selected for testing of Staphylococcus aureus). However, the inclusion of this data does not impact significantly the share of these applications in the total consumption amount.

AF5
 Estimates of health and health care related statistics are based on the most recent data available in the public domain, except for incidences of Venous Thomboembolism for the UK.
 In most cases, dose of the therapeutic agent is used to arrive at estimates and the gold amounts that would be present in drug delivery equipments, containers containing the drug, etc. have not been included in our estimates.
 Census data of the US (2010) and UK (2011) have been used to arrive at the prospective population. The details of the data and assumptions used to calculate annual consumption of Au-NP from medical applications selected for the study is presented below. The two step approach: 1. Estimate the range of nano gold amount per application 2. Estimate the prospective affected population/total number of tests AF6

Assumptions for annual total number of tests
Since this is a home based test based on oral fluids, we assume 50% of people from age 15 to 64 years conduct one home based HIV test per year, though legally the self-testing kit is to be sold to population aged 17 years or more, we have used 15-64 yrs because of the class intervals provided in the population tables. Mass gold /ml = 5.69*10 -5 g/ml [4] 3. 15 µl of gold conjugate solution per strip [5] 4. Two test kits per test [16]. Therefore, 30 µl of gold conjugate per test, i.e., 0.03ml = 1.7*10 -5 g of Au per test device Assumptions for annual total number of tests US: No. of discharges with septicaemia = 1665400. Around 15% (approx 250000) of the above discharges were diagnosed to be due to gram positive bacteria [17] 50% of patients suffering from septicaemia, the bacteria is unspecified. And, 15% have bacteria present in blood, but without the response. Keeping these factors into consideration, assume 30% more tests to be done. [17] Therefore, total no. of tests = 25000 + 30% of 250000. ca.325000

UK:
No. of MSSA and MRSA reports in England (above 2 years of age) year 2013 = ca. 10000 [18] Population for England above 4 years is ca. 50 million[9].

Total population over 4 yrs for UK = ca. 60 million[9]
So, for the UK = estimated number of MRSA and MSSA cases is 12000 (above 4 years of age) approx. = assume 15000 for all age groups. Therefore, total no. of tests = 15000 + 30% more tests = 15000 + 4500 = ca. 20000

G.1.3. C. difficile infections (CDI)
336, 600 hospitalizations that involved CDI in 2009 [48] Assume 10% more diagnostic tests have been performed So, no. of tests/year for the US = 10% of 336600 +336600 = 370260 For England, reported cases is 13756 for the year 2013 [18] To estimate reported cases for CD infections for the UK, using the rate of 30 per 100000 of population = 18955 [18] Assume 10% more tests conducted

G.2. Test kit for detection of single nucleotide polymorphism (F2/F5) to establish risk from venous thrombosis (VTE)
Assumptions to estimate amount of Au per application 1. Au-NP size = 13-20 nm [47]; assume Au-NP size = 20 nm 2. Volume per test cartridge: 0.1 ml, i.e., ca. 2 drops 3. Mass of gold per ml = 5.66*10 -5 g [4]; mass of gold in 0.1 ml or mass of Au per application = 5.66*10 -6 g Assumptions for annual total number of tests 1. Prevalence of Factor V Leiden in European Whites = 3-15% [50] 2. Prevalence of Factor V Leiden in UK = 8.8% [50] 3. Prevalence of Factor V Leiden in Unites States, white population = 5.2% [50] Assume, 8% of the white population will carry Factor V gene mutation US white population = 223553265 9 = 8% of 223553265 = 17884261 White population for England and Wales = 54809000 [51] = 8% of 54809000 = 4384720 = approx. 4400000 Estimated annual average of hospitalizations with VTE (≥18 years in the United States) = 547596 among those aged ≥18 years in the United States [52] 547596 hospitalisations shows 3% of the white population of the US who might carry one of the risk factors for VTE are hospitalised in a given year. Therefore, we assume 5% of the white population of the US and UK gets the genetic test done. 5% of 4400000 for the UK = approx. 225000

G.3. Test kit for detection and genotyping Warfarin metabolism
Assumptions to estimate amount of Au per application 1. Au-NP size = 13-20 nm [47]; assume Au-NP size = 20 nm 2. Volume per test cartridge: 0.1 ml, i.e., ca. 2 drops 3. Mass of gold per ml = 5.66*10 -5 g [4]; mass of gold in 0.1 ml or mass of Au per application = 5.66*10 -6 g Assumptions for annual total number of tests  [58] 6. Since the available literature doesn't inform of the number of PEG on one Au-NP [61].
Assume, both SH-PEG and rhTNF are bound to the Au-NP and they do not cross-link with each other.

Type of cancer US UK
Estimated cases in 2014 [45] Estimated deaths for 2014 [45] Diagnosed Cases for 2011 Deaths in 2012 Colorectal 136830 50310 41581 [62] 16187 [62] Pancreatic 46420 39590 8773 [63] 8662 [63] Breast 235630 40430 50285 [64] 11716 [ (120 nm diameter is the silica core) with a coating of polyethylene glycol 5000. Estimates of Amount of Au per patient  95% of (21*70*2) =2793mg  95% of (35*70*2) = 4655mg Population assumptions for annual total consumption Type of cancer US UK Estimated cases in 2014 [45] Estimated deaths for 2014 [45] Diagnosed  [67] 2119 (oral cancer) [ [70] Therefore, assume 30% of the diagnosed population of all age groups take insulin = 6.3 million UK =3.2 million people have been diagnosed with diabetes (2013) [71] 10 http://www.nanospectra.com/clinicians/trialinfo.html: The clinical trials include metastatic lung cancer and refractory head and neck cancer 11 http://dtc.ucsf.edu/types-of-diabetes/type1/treatment-of-type-1-diabetes/medications-and-therapies/type-1-insulin-therapy/calculating-insulin-dose/ AF20 Also, assume 30% of UK's diabetic patients will take insulin (as derived from the American numbers) = 30% of 3.2 million = 960000 AF21 Table AF.T3.1 Summary of volume or mass of environment compartmentair, water, sediment and soilas input parameters for the probabilistic mass flow model. The Comments column provides the values used to calculate the mass/volume. The mass of soil and sediment compartment has been arrived at by multiplying the area, the mixing depth and the density. The area of natural and urban soils has been calculated by subtracting the area occupied by agricultural soils and other soils. Littoral sediments (beaches and intertidal mud flats and salt marshes) have been included for the UK as it represents a key ecosystem of the UK

Compartments Countries Formula/ Calculation Mass/Volume Unit Comments
Sludge treated soils UK 1.65*10 9 *0.2*1.5*10 3 4.95E+11 kg  1.65*10 9 m 2 : total sludge treated agricultural land area in the UK [72]  0.2 m: the depth of agricultural soil [73]  408,139,000 acres is the total cropland/arable land in the US 12 = ca.1.65*10 12 m 2 [74,75]  Total area of sludge treated soil the US: 1% [76] of arable land= 1.65*10 10 m 2  1.5*10 3 kg/m 3 : the density of dry soil [ 12 1 acre = 0.004046 km 2 13 In the US, treated sewage sludge is termed as biosolids.  The higher estimate of 5.35 million tonnes has been used in the study:  Due the futuristic perspective of nanomedicine waste  Increasing stringency in regulations concerning hospital waste AF27