In December 2019, several cases of pneumonia with unknown etiology started to 49 arise in Wuhan, China. A new betacoronavirus was identified and named SARS-CoV-2 50 due to high similarity with previous SARS-CoV 1,2 . This virus causes the disease which 51 has been called COVID-19 3 .Since then, SARS-CoV-2 has rapidly spread worldwide 52 prompting the World Health Organization to declare the outbreak a pandemic, with more 53 8
The first criterion for the applicability assessment is determined based on 137 whether it fits within the range of the key molecular descriptors of the training set (MW, 138 MolLogP, NRB, TPSA, HBA, HBD). If at least four properties lie within the maximum 139
and minimum values of the model's data, the molecule is considered similar and goes to 140 the next criterion. The second criterion relies on structural fragment-based similarity 141 measured with Tanimoto coefficient using MACCS fingerprints. The similarity of the 142 MACCS fingerprints for the query compound and all training data is computed using the 143 Tanimoto score. Only 5% of the training set compounds that are most similar to the 144 query compound is used for evaluation (i.e. if the training set has 100 molecules only 5 145 molecules with more similarity to the query compound are used for the next evaluation). 146
If the Tanimoto score exceeds 0.5 against the 5% of the training set compounds, the 147 model is considered to have enough structural fragments overlap with the query 148 compound and thus the compound goes onto the reliability assessment. 149
The reliability domain assessment implements k-means clustering methods 150 based on ECFC6 fingerprints to classify the predictions from very high to low reliability. 151
The reliability class depends on four criteria: distance from the major central point of the 152 training data, distance from the closest cluster, closest cluster density and closest 153 cluster distance within the chemical space. Each criterion has different weights and 154 scores, with the second and third having higher priority. If the compound scores 1 in 155 each criterion it is classified as very highly reliable, if that is not the case only the two 156 higher priority criteria are considered for the next classes. The compound is classified 157 as highly reliable if scores a total of 2, moderately reliable if it scores between -1 and 2 158 or low reliability if it scores less than or equal to -1 in the two higher priority criteria. The 9 scores for each criterion as well as its definition are extensively described in the 160 Supplemental Methods. 161 162
Compounds were tested in a 10-point serial dilution experiment to determine the 50% 164 inhibitory concentration (IC 50 ) and 50% cytotoxicity concentration (CC 50 ). 1,000 HeLa-165 ACE2 cells/well were added into 384-well plates with compounds in a volume of 25 nl. 
The performance of the machine learning models on the external testing data is 207
shown in Table 2 . The external validation was used to measure model performance in 208 data from different studies outside the training set. svc and knn had slightly better 209 statistics compared to all other models, with the best balance between recall and 210 specificity. 211 212 
The PCA of the model training set alone shows that the SARS-CoV-2 chemical 218 space is well distributed with active and inactive molecules well mixed when analyzed 219 using either molecular and fingerprint descriptors. When compared with Prestwick 220
Chemical Library (PwCL), a library of predominantly FDA approved drugs, the SARS-221
CoV-2 data lie within a big cluster with molecular descriptors and is more widely 222 distributed when using the fingerprint descriptors. suggesting it is likely more conservative. Indeed, with the external test and training set 244
PCA we can see that most molecules superimpose with few of them distant from each 245 other ( Figure S1 ). Therefore, similarity together with clustering methods are more 246 suitable for applicability and reliability assessment compared with only structural 247 similarity, as seen by the PCA. 248 249
A selection of FDA approved drugs available to us in our relatively small in-house 251 compound collection of hundreds of molecules was scored with the AC Bayesian model. 252
A selection of some of the best scoring molecules (Table 3) (Table 1) . When compared with different machine learning methods AC 282 outperforms all of them in the SARS-CoV-2 training set, but this may be due to the 283 threshold for all models being set as optimal for AC. However, choosing different values 284 could imbalance the training set and remove important compounds from the active 285
More important than a good performance in the training set is the performance 287 on external data, since most prospective predictions will occur for molecules outside 288 training data. For external validation all models had intermediate performance, with 289 ROC of 0.6. Taking into account the small number of molecules and that some test set 290 molecules lie outside the applicability domain, the performance is acceptable. Different 291 from the training set performance, svc had the highest overall score, predicting 60% of 292 the active molecules despite its modest statistics in five-fold cross validation. The good 293 performance of svc in predicting biological activity is in accordance with several studies 294 that show good performance in different datasets 28,32,35,39 . Therefore, the models 295 described here are suitable for initial prospective predictions. 296
The applicability and reliability assessment shows that 73% of the test set The molecules of the dataset do not have a common scaffold, but there are 314 several common structural features that occur in active/inactive molecules that can be 315 highlighted, such as tertiary amines and aliphatic chains in active molecules and phenyl 316 rings and peptide molecule features in inactive molecules ( Figure S3 ). These most 317 common active features appear in chloroquine, tripanarol and tilorone, while the inactive 318 features appear in darunavir, amprenavir and ritonavir (Figure 3 ). The lack of common 319 scaffolds and features that appears in more than 30% of the active or inactive 320 molecules shows how different and diverse the active molecules are, which turn 321 classification models for these molecules into a relatively difficult task. or not the expression of TMPRSS2 affects compound activity. 45 357
As new data is continually being published the machine learning models can be 358 updated to increase performance in terms of both training and external test set 359 validation. The very latest model for SARS-CoV-2 is available at www.assaycentral.org. 360
In the meantime, we have shown these models perform well with internal cross 361 validation, external validation as well as prospective prediction, enabling us to find 362 additional active molecules. These models should be used to prioritize compounds 363 which have both a high prediction score and reliability as described herein. This will be 364 expected to return more reliable predictions that together with drug discovery expertise 365 can help prioritize compounds in future for in vitro testing. 366 367
We would like to kindly acknowledge Dr. Nancy Baker and Ms. Natasha Baker for their 369 help in collating recently SARS-CoV-2 published data. We also thank Biovia for 